Tortoise and Turtle

Tortoises are a family, Testudinidae, of land-dwelling reptiles in the order Testudines. Tortoises are shielded from predators by a shell. The top part of the shell is the carapace, the underside is the plastron, and the two are connected by the bridge. The carapace is fused to both the vertebrae and ribcage, and tortoises are unique among vertebrates in that the pectoral and pelvic girdles are inside, rather than outside, the ribcage. Tortoises can vary in size from a few centimeters to two meters. They are usually diurnal animals with tendencies to be crepuscular depending on the ambient temperatures. They are generally reclusive animals.

Turtles are reptiles of the order Testudines (or Chelonii) characterised by a special bony or cartilaginous shell developed from their ribs and acting as a shield.[4] "Turtle" may refer to the order as a whole (American English) or to fresh-water and sea-dwelling testudines (British English).

Differences exist in usage of the common terms turtle, tortoise, and terrapin, depending on the variety of English being used; usage is inconsistent and contradictory. These terms are common names and do not reflect precise biological or taxonomic distinctions.
The American Society of Ichthyologists and Herpetologists uses "turtle" to describe all species of the order Testudines, regardless of whether they are land-dwelling or sea-dwelling, and uses "tortoise" as a more specific term for slow-moving terrestrial species. General American usage agrees; turtle is often a general term (although some restrict it to aquatic turtles); tortoise is used only in reference to terrestrial turtles or, more narrowly, only those members of Testudinidae, the family of modern land tortoises; and terrapin may refer to turtles that are small and live in fresh and brackish water, in particular the diamondback terrapin (Malaclemys terrapin). In America, for example, the members of the genus Terrapene dwell on land, yet are referred to as box turtles rather than tortoises.
(Wikipedia)

Illustration depicting organisms classified as Chelonia (turtles)

Illustration depicting organisms classified as Chelonia (turtles). The 89th plate from Ernst Haeckel's "Kunstformen der Natur" (Art forms of Nature) (1904),
Haeckel Chelonia Haeckel Chelonia big spots

1: Leatherback Turtle -- Dermatochelys coriacea (Blainville) = Dermochelys coriacea (Vandelli, 1761)
2: Hawksbill Turtle -- Caretta imbricata (Gray) = Eretmochelys imbricata (Linnaeus, 1766)
3: Argentine Snake-necked Turtle -- Hydromeda tectifera (Wagler) = Hydromedusa tectifera Cope, 1869
4: Mata mata -- Chelys fimbriata (Duméril) = Geochelone nigra (Quoy & Gaimard, 1824)
5: Geometric Tortoise -- Testudo geometrica (Linné) = Psammobates geometricus (Linnaeus, 1758)
6: Galápagos Tortoise -- Testudo elephantina (Duméril) = Dipsochelys dussumieri Gray, 1831
7: Common Snapping Turtle -- Chelydra serpentina (Schweigger) = Chelydra serpentina (Linnaeus, 1758)

Tortoises in art

Left: 'Jurōjin, Deer and Tortoises in a Landscape' by Shibata Zeshin, 1889, ink and color on silk, Honolulu Museum of Art.
Right: 'Tortoise on a River Bank' by Shibata Zeshin, c. 1873-91, ink on paper, Honolulu Museum of Art.

Image sources: commons.wikimedia.org commons.wikimedia.org

Angonoka tortoise (Astrochelys yniphora)

The angonoka tortoise (Astrochelys yniphora) is a critically endangered species of tortoise endemic to Madagascar. It is also known as the angonoka, ploughshare tortoise, Madagascar tortoise, or Madagascar angulated tortoise (Wikipedia).

This species is one of the rarest land tortoises in the world. Its common name refers to the appearance of the 'gular scute' at the lower part of the shell (plastron), which is drawn out into a plough-shaped projection between the front legs. The upper shell (carapace) is hard, highly domed and brown in colour, with prominent concentric growth rings on each scute. Males are larger than females. (eol.org)

Left: Angonoka tortoise Endemic Madagascan tortoise, Here at Pairi Daiza, Brugelette, Belgium
Right: The ploughshare tortoise

Image sources: commons.wikimedia.org www.cnn.com

Angonoka tortoise (Astrochelys yniphora)

Angonoka tortoise Angonoka tortoise

Image sources: Multiple sources Multiple sources

Angulate tortoise (Chersina angulata)

The angulate tortoise (Chersina angulata) is a species of tortoise found in dry areas and coastal scrub vegetation in South Africa. This species is highly distinctive and is now classified by itself, in the monotypic Chersina genus.
Identification: A small, shy tortoise with a relatively variable shell, they can often be distinguished by their prominent "bowsprits", which are protrusions of the "gular" shields, from their plastrons under their chins. These are used by males to fight for territory or females. Uniquely, this species has only one gular shield under its chin; all other southern African tortoises have a divided/double scaled gular. Angulate specimens have 5 claws on their front legs and 4 on each back leg. They also, like most other southern African tortoises, have a nuchal scute. (Wikipedia).

Left: Two male Chersina angulata tortoises a male Chersina angulata tortoise demonstrating courting behaviour to another male. Cape Town. South Africa.
Right: Female Chersina angulata tortoise in Cape Town.

Image sources: commons.wikimedia.org commons.wikimedia.org

Left: Adult male Chersina angulata
Right: Chersina angulata (Illustration) In "Tortoises, terrapins, and turtles" : drawn from life / by James de Carle Sowerby and Edward Lear..

Image sources: Left: photobucket.com (www.tortoiseforum.org) Right: commons.wikimedia.org (flickr.com) (biodiversitylibrary.org)

Radiated tortoise (Astrochelys radiata)

The radiated tortoise (Astrochelys radiata) is a species in the family Testudinidae. Although this species is native to and most abundant in southern Madagascar, it can also be found in the rest of this island, and has been introduced to the islands of Réunion and Mauritius. It is a very long-lived species, with recorded lifespans of at least 188 years. These tortoises are classified as Critically Endangered by the IUCN, mainly because of the destruction of their habitat and because of poaching. (Wikipedia).

Left: Radiated Tortoise (Astrochelys radiata) Arboretum d 'Antsokay, Toliara, Madagascar. (Botanical garden in Madagascar)
Right: Radiated Tortoise (Astrochelys radiata) Toliara, Madagascar.

Image sources: commons.wikimedia.org (www.flickr.com) commons.wikimedia.org (www.flickr.com)

Geometric tortoise (Psammobates geometricus)

The geometric tortoise is a critically endangered species of tortoise and one of three members of the genus, Psammobates. It is found in a very small section in the South-Western Cape of South Africa.
It has a very strong, black and yellow patterned carapace, used for defense against predators. The patterns are arranged in ray-like markings and help the tortoise blend in with its environment. From a birds eye view the shell has geometrical symbols on it thus giving it its name. This tortoise is very small, and a full grown tortoise can only reach about 5 to 6 inches in diameter. (Wikipedia).

Geometric tortoise

Image source: The Tortoise Farm Nature Reserve (wildlifeforall.org)

Left: Psammobates geometricus Psammobates geometricus has lost 97 per cent of its habitat.
Right: Psammobates geometricus (Illustration) In "Tortoises, terrapins, and turtles" : drawn from life / by James de Carle Sowerby and Edward Lear.

Image sources: commons.wikimedia.org commons.wikimedia.org (biodiversitylibrary.org)

Left: Psammobates geometricus (Illustration) Crop detail of the 89th plate from Ernst Haeckel's "Kunstformen der Natur" (Art forms of Nature) (1904)
Right: Psammobates geometricus (Illustration) Detail of the 89th plate from Ernst Haeckel's "Kunstformen der Natur" (Art forms of Nature) (1904)

Image sources: commons.wikimedia.org commons.wikimedia.org

Mollusca

The molluscs (or mollusks) compose the large phylum of invertebrate animals known as the Mollusca. Around 85,000 extant species of molluscs are recognized. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat. The phylum is typically divided into 9 or 10 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish and octopus, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs in terms of classified species, and account for 80% of the total. The scientific study of molluscs is called malacology.
The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula (except for bivalves), and the structure of the nervous system. Other than these things, molluscs express great morphological diversity. (Wikipedia)

Classification
Opinions vary about the number of classes of molluscs; for example, the table below shows eight living classes, and two extinct ones. Although they are unlikely to form a clade, some older works combine the Caudofoveata and solenogasters into one class, the Aplacophora. Two of the commonly recognized "classes" are known only from fossils.

Caudofoveata, Solenogastres, Polyplacophora, Monoplacophora, Gastropoda, Cephalopoda, Bivalvia, Scaphopoda, Rostroconchia, Helcionelloida.

Cephalopod (頭足類動物)

A cephalopod is any member of the molluscan class Cephalopoda. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles (muscular hydrostats) modified from the primitive molluscan foot. Fishermen sometimes call them inkfish, referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology.
Cephalopods became dominant during the Ordovician period, represented by primitive nautiloids. The class now contains two, only distantly related, extant subclasses: Coleoidea, which includes octopuses(章鱼), squid (烏賊), and cuttlefish (墨魚, 鱿魚); and Nautiloidea, represented by Nautilus and Allonautilus. In the Coleoidea, the molluscan shell has been internalized or is absent, whereas in the Nautiloidea, the external shell remains. About 800 living species of cephalopods have been identified. Two important extinct taxa are the Ammonoidea (ammonites) and Belemnoidea (belemnites).
Cephalopod (頭足類動物) - 觸鬚與頭部直接相連

What are Cephalopods?

Squid

Squid are cephalopods of the order Teuthida, which comprises around 304 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles. Squid are strong swimmers and certain species can "fly" for short distances out of the water.

Caribbean Reef Squid (Sepioteuthis sepioidea)

Sepioteuthis sepioidea, the Caribbean Reef squid, is commonly observed in shallow near shore water of the Caribbean by divers and snorkelers. These squid are often confused with cuttlefish because they have large fins that wrap around their mantles. They are also broader than many squid and these traits make them look like cuttlefish (Sepia). Actually, if you look at their scientific name you will see that scientists also noticed that these guys look like cuttlefish—the "Sepio" which is in the genus and specific name refers to the fact that they look like cuttlefish while the "teuthis" tells one that they really are a squid. For the record, there are no true cuttlefish (Order Sepiida) off of North America although there are some Sepiolids. (www.thecephalopodpage.org)

Caribbean reef squid (Sepioteuthis sepioidea), commonly called reef squid, are small 20 cm (7.9 in) torpedo-shaped squid with undulating fins that extend nearly the entire length of the body. In 2001, marine biologist Silvia Maciá discovered that squid were able to propel themselves up out of the water about 2 m (6.6 ft) and fly approximately 10 m (33 ft) before re-entry; a discovery which led to the identification of six species of flying squid. (Wikipedia)

Caribbean Reef Squid

Left: Caribbean Reef Squid (Sepioteuthis sepioidea) on Bari Reef, Bonaire, BES Islands
Right: Reef squid (Sepioteuthis sp., Bonaire)

Sepioteuthis sepioidea (Caribbean Reef Squid)

Sepioteuthis sepioidea (Caribbean Reef Squid) hovering just above seafloo. (La Fague, Cap-Haitien, Haiti)

Reef squid

Caribbean Reef Squid (Sepioteuthis sepioidea) Bonaire.

Close-up of a Caribbean reef squid

Image source: commons.wikimedia.org

Bigfin reef squid (Sepioteuthis lessoniana)

Sepioteuthis lessoniana, commonly known as the bigfin reef squid or oval squid, is a commercially important species of loliginid squid. It is one of the three currently recognized species belonging to the genus Sepioteuthis. Studies in 1993, however, have indicated that bigfin reef squids may comprise a cryptic species complex. The species is likely to include several very similar and closely related species.
Bigfin reef squids are characterised by a large oval fin that extends throughout the margins of its mantle, giving them a superficial similarity to cuttlefish. They are small to medium-sized squids, averaging 3.8 to 33 centimetres (1.5 to 13.0 in) in length. (Wikipedia)

Bigfin reef squid (Sepioteuthis lessoniana)

Sepioteuthis lessoniana at Kinosaki Marine World, Japan

Bigfin reef squid (Sepioteuthis lessoniana)

Probably Sepioteuthis lessoniana at Komodo National Park, Indonesia.

Close-up of Bigfin Reef Squid (Sepioteuthis lessoniana)

Close-up of Bigfin Reef Squid (Sepioteuthis lessoniana) at Voavah, Baa Atoll in Maldives

Image source: flickr.com commons.wikimedia.org

Juvenile bigfin reef squid at Monterey Bay Aquarium.

Juvenile bigfin reef squid at Monterey Bay Aquarium

Bigfin reef squid at Monterey Bay Aquarium.

Bigfin reef squid at Monterey Bay Aquarium

Caribbean reef squid (Sepioteuthis sepioidea)

Squid at Sunset Reef Grand Cayman, Cayman Islands

Image source: www.today.com Multiple sources

Bigfin reef squid (Sepioteuthis lessoniana)

Sepioteuthis lessoniana at Voavah, Baa Atoll in Maldives

Image source: commons.wikimedia.org

Squid (Teuthida)

Squid (mollusca, cephalopoda, teuthida)

Loligo vulgaris

Loligo vulgaris Photo of squid with 8 short arms and two longer tentacles. (Common squid from the Belgian continental shelf)

Image source: commons.wikimedia.org

Cephalopods are Quick-Change Artists

See it on youtube.com See it on montereybayaquarium.org

Cephalopods are Clever Hunters

See it on youtube.com

Cuttlefish

Cuttlefish are marine animals of the order Sepiida. They belong to the class Cephalopoda. Cuttlefish have a unique internal shell, the cuttlebone. Despite their name, cuttlefish are not fish but molluscs.
Cuttlefish have large, W-shaped pupils, eight arms, and two tentacles furnished with denticulated suckers, with which they secure their prey. They generally range in size from 15 to 25 cm (5.9 to 9.8 in), with the largest species, Sepia apama, reaching 50 cm (20 in) in mantle length and over 10.5 kg (23 lb) in mass.

Sepia officinalis (Common cuttlefish)

Sepia officinalis at Nausicaä Centre National de la Mer, Boulogne-sur-Mer, France The common cuttlefish or European common cuttlefish (Sepia officinalis) is one of the largest and best-known cuttlefish species. It grows to 49 cm in mantle length (ML) and 4 kg in weight.

Giant Cuttlefish-Sepia apama

Australian giant cuttlefish (Sepia apama) at Clovelly, Sydney, NSW.

Sepia latimanus

Sepia latimanus colour change. These images were taken only seconds apart.
Left: Sepia latimanus (Reef cuttlefish) dark coloration. Seconds before turning white - in right image
Right: Sepia latimanus (Reef cuttlefish) white coloration. Seconds prior this individual was dark in coloration as in left image.

The camouflage of a cuttlefish

View Cuttlefish Video at YouTube.com
World's Deadliest - "Sudden Death" Cuttlefish

Octopus

The octopus (plural: octopuses, octopodes or octopi) is a cephalopod mollusc of the order Octopoda. It has two eyes and four pairs of arms and, like other cephalopods, it is bilaterally symmetric. It has a beak, with its mouth at the center point of the arms. It has no internal or external skeleton (although some species have a vestigial remnant of a shell inside their mantles), allowing it to squeeze through tight places. Octopuses are among the most intelligent and behaviorally diverse of all invertebrates.
Octopuses inhabit diverse regions of the ocean, including coral reefs, pelagic waters, and the ocean floor. They have numerous strategies for defending themselves against predators, including the expulsion of ink, the use of camouflage and deimatic displays, their ability to jet quickly through the water, and their ability to hide. They trail their eight arms behind them as they swim. All octopuses are venomous, but only one group, the blue-ringed octopus, is known to be deadly to humans.
Around 300 species are recognized, which is over one-third of the total number of known cephalopod species. The term 'octopus' may also be used to refer specifically to the genus Octopus.
(Wikipedia)

The common octopus (Octopus vulgaris)

The common octopus (Octopus vulgaris)
The common octopus (Octopus vulgaris) is a mollusc belonging to the class Cephalopoda. The animal belongs to the Superphylum Lophotrochozoa, being considered a protostome and having thus a coelom. Octopus vulgaris is the most studied of all octopus species. (Wikipedia)

Common octopus (Octopus vulgaris)

Common octopus (Octopus vulgaris) Staatliches Museum für Naturkunde Karlsruhe, Germany.

Hawaiian Octopus

Octopus paralarva

Octopus paralarva
Paralarvae (singular: paralarva) are young cephalopods in the planktonic stages between hatchling and subadult. This stage differs from the larval stage of animals that undergo true metamorphosis. Paralarvae have been observed only in members of the orders Octopoda and Teuthida. (Wikipedia)

Ghostlike Octopod

Ghostlike Octopod This ghostlike octopod is almost certainly an undescribed species and may not belong to any described genus.
Image courtesy of NOAA Office of Ocean Exploration and Research, Hohonu Moana 2016.

The appearance of this animal was unlike any published records and was the deepest observation ever for this type of cephalopod.
Deep-sea octopods are easily separated into two distinct groups: (1) the cirrate, or finned, octopods (also known as “dumbo” octopods), characterized by fins on the sides of their bodies and fingerlike cirri associated with the suckers on their arms and (2) incirrate octopods, which lack both fins and cirri and are similar in appearance to common shallow-water Octopus.
The octopod imaged in detail on this first dive was a member of the second group, the incirrates. A distinctive characteristic was that the suckers were in one, rather than two, series on each arm. This animal was particularly unusual because it lacked the pigment cells, called chromatophores, typical of most cephalopods, and it did not seem very muscular. This resulted in a ghostlike appearance, leading to a comment on social media that it should be called Casper, like the friendly cartoon ghost. It is almost certainly an undescribed species and may not belong to any described genus.

Video of Ghostlike Octopod

Ghostlike Octopod

Gastropoda

The Gastropoda or gastropods, more commonly known as snails and slugs, are a large taxonomic class within the phylum Mollusca. The class Gastropoda includes snails and slugs of all kinds and all sizes from microscopic to large. There are many thousands of species of sea snails and sea slugs, as well as freshwater snails, freshwater limpets, land snails and land slugs.

In the Bouchet, Rocroi et al. taxonomy, the authors have used unranked clades for taxa above the rank of superfamily (replacing the ranks suborder, order, superorder and subclass), while using the traditional Linnaean approach for all taxa below the rank of superfamily. Whenever monophyly has not been tested, or is known to be paraphyletic or polyphyletic, the term "group" or "informal group" has been used. The classification of families into subfamilies is often not well resolved, and should be regarded as the best possible hypothesis.
(Wikipedia)

Nudibranch

Nudibranchs are a group of soft-bodied, marine gastropod molluscs which shed their shells after their larval stage. They are noted for their often extraordinary colours and striking forms. Currently, about 2,300 valid species of nudibranchs are known.
The word "nudibranch" comes from the Latin nudus, naked, and the Greek βραγχια, brankhia, gills.
Nudibranchs are often casually called sea slugs, but many sea slugs belong to several taxonomic groups which are not closely related to nudibranchs. A number of these other sea slugs, such as the photosynthetic Sacoglossa and the colourful Aglajidae, are often confused with nudibranchs.

Nudibranchs are commonly divided into two main kinds, dorid and aeolid nudibranchs:
Dorids are recognised by the branchial (gill) plume, which forms a cluster on the posterior part of the body, around the anus. Fringes on the mantle do not contain any intestines.
Aeolids have cerata (spread across the back) instead of the branchial plume. They lack a mantle. Some are hosts to zooxanthellae.

According to the taxonomy by Bouchet & Rocroi (2005), the Nudibranchia are a subclade within the clade of the Nudipleura. The Nudibranchia are then divided into two clades: Euctinidiacea and Dexiarchia.
*Euctinidiacea (Holohepatica) includes Gnathodoridacea (contains only Bathydorididae) and Doridacea (contains Doridoidea, hyllidioidea, Onchidoridoidea, Polyceroidea ).
*Dexiarchia (Actenidiacea) includes Pseudoeuctenidiacea (Doridoxida) and Cladobranchia (Cladohepatica).
(Wikipedia)

Anatomy of Dorid nudibranchs

Anatomy of Dorid nudibranchs
In typical dorid nudibranchs, the mantle is thick and extends over the foot. The surface of the mantle may bear tubercles which vary in size, shape and number and are often a character used to identify nudibranchs. In many dorids acid glands and/or spicules are incorporated in the mantle tissue and it is thought that these are mainly defensive in function. However in Goniodorids, Polycerids and some other dorid families the mantle is progressively reduced to a ridge around the side of the body, from which pallial tentacles or processes arise. These processes usually have coloured tips and contain defensive glands and have been shown to produce chemicals distateful to fish. These chemicals are often manufactured from similar chemical compounds in the bryozoan or ascidian prey, or may be the same molecules selectively re-secreted by the nudibranch. (www.seaslug.org.uk)

Anatomy of Aeolid nudibranchs

Anatomy of Aeolid nudibranchs
In aeolid nudibranchs the mantle is extended into long finger-like projections called cerata (singular; ceras). The cerata contain branches of digestive gland and often this is visible through the ceratal epidermis. In aeolids the tips of the cerata contain cnidosacs which usually store nematocysts (stinging cells) that are obtained from ingested cnidarian prey, such as hydroids, sea-anemones and soft corals. If disturbed, the nudibranch is capable of discharging these stinging cells through a terminal pore in the ceras; this is an effective deterrent to predatory fish. (www.seaslug.org.uk)

Dorid nudibranchs

Superfamily: Doridoidea

Doridoidea, commonly known as dorid nudibranchs (and previously known as the taxon Cryptobranchia), are a taxonomic superfamily of medium to large, shell-less sea slugs, marine gastropod mollusks in the clade Doridacea.
Families within the superfamily Doridoidea include: Actinocyclidae, Chromodorididae, Discodorididae, Dorididae.

Cadlinella (Superfamily: Doridoidea, Family: Chromodorididae, Genus: Cadlinella)
Left: Cadlinella ornatissima 13 mm, Sri Lanka. Cadlinella ornatissima is a species of sea slug or dorid nudibranch, a marine gastropod mollusk in the family Chromodorididae.
Middle Left: Cadlinella ornatissima in Lizard Island, Australia
Middle Right: Cadlinella ornatissima Marine Park Izu, Japan
Right: Cadlinella sagamiensis 43 m, in the Marine Park Izu, Japan

The Chromodorididae, or chromodorids, are a taxonomic family of colorful, sea slugs; dorid nudibranchs, marine gastropod mollusks in the superfamily Doridoidea.
Cadlinella is a genus of colorful sea slugs, specifically dorid nudibranchs, shell-less marine gastropod mollusks in the family Chromodorididae.

Doriprismatica atromarginata (Glossodoris atromarginata)

Doriprismatica atromarginata (Superfamily: Doridoidea, Family: Chromodorididae, Genus: Doriprismatica)
Left: Doriprismatica atromarginata (Glossodoris atromarginata)
Middle Left: Doriprismatica atromarginata (Glossodoris atromarginata)
Middle Right: Doriprismatica atromarginata Koh Phangan, Tailandia
Right: Doriprismatica atromarginata at Bima Bay (Sumbawa, Indonesia)

Doriprismatica atromarginata is a species of sea slug, a dorid nudibranch, a shell-less marine gastropod mollusk in the family Chromodorididae.

Mexichromis multituberculata (Superfamily: Doridoidea, Family: Chromodorididae, Genus: Mexichromis)
Left: Mexichromis multituberculata
Middle Left: Mexichromis multituberculata
Middle Right: Mexichromis multituberculata
Right: Mexichromis multituberculata

Mexichromis multituberculata is a species of sea slug, a dorid nudibranch, a shell-less marine gastropod mollusk in the family Chromodorididae.

Peltodoris atromaculata (Doris dalmatien)

Left: Peltodoris atromaculata (Superfamily: Doridoidea, Family: Discodorididae, Genus: Peltodoris)
Middle Left: Peltodoris atromaculata (Doris dalmatien)
Middle Right: Taringa halgerda (Superfamily: Doridoidea, Family: Discodorididae, Genus: Taringa) taken near Puerto Galera, the Philippines
Right: Doris odhneri (Superfamily: Doridoidea, Family: Dorididae, Genus: Doris) Doris odhneri is a species of sea slug, a dorid nudibranch, a shell-less marine gastropod mollusk in the family Dorididae. It is known by many common names, such as: giant white nudibranch, giant white dorid, and white-knight nudibranch.

Peltodoris atromaculata is a species of sea slug, a dorid nudibranch, a marine gastropod mollusk in the family Discodorididae.

Felimare picta (Superfamily: Doridoidea, Family: Chromodorididae, Genus: Felimare)
Left: Felimare picta at Gray's Reef National Marine Sanctuary, Savannah, Georgia
Middle Left: Felimare picta on wreck of SS Rosslyn, Gibraltar.
Middle Right: Felimare picta
Right: Felimare picta (synonym: Hypselodoris edenticulata) Gray's reef, Georgia, EE.UU.

Felimare picta is a species of colorful sea slug or dorid nudibranch, a marine gastropod mollusk in the family Chromodorididae.

Spanish dancer (Hexabranchus sanguineus)

(Clade Nudibranchia→Euctenidiacea→Doridacea→Superfamily Polyceroidea→Family Hexabranchidae→Genus Hexabranchus.)
The Spanish dancer, scientific name Hexabranchus sanguineus (literally meaning "blood-colored six-gills"), is a dorid nudibranch, a very large and colorful sea slug, a marine gastropod mollusk in the family Hexabranchidae. (Wikipedia)

Hexabranchus sanguineus, the anterior end is on the top and the six-gills on the bottom.
Left: Hexabranchus sanguineus
Middle Left: Hexabranchus sanguineus 90-centimeter (35-inch) giant Spanish Dancer (Hexabranchus sanguineus) in the hull of Liberty Wreck at night.
Middle Right: A swimming Hexabranchus sanguineus
Right: Hexabranchus sanguineus

Left: Spanish dancer (Hexabranchus sanguineus) encountered on the North coast of East Timor. This individual was approximately 10 inches long.
Middle Left: Spanish Dancer (Hexabranchus sanguineus) Taken in the Red Sea at low tide, the coral reef from the Hotel Grand Makadi in Makadi Bay.
Middle Right: Hexabranchus sanguineus
Right: Hexabranchus sanguineus

Juvenile Hexabranchus sanguineus. Juveniles usually bear spotting, which disappears as the animal grows.
Left: Hexabranchus sanguineus, juvenile 13 mm, Maldives
Middle Left: Hexabranchus sanguineus, sub-adult
Middle Right: Juvenile Hexabranchus sanguineus
Right: Juvenile Hexabranchus sanguineus

Video of a swimming Spanish Dancer (Hexabranchus sanguineus)

Dexiarchia

The Dexiarchia are a suborder of sea slugs, shell-less marine gastropod molluscs in the order Nudibranchia. This classification is based on the study by Schrödl et al., published in 2001, who recognized within this clade two clades Pseudoeuctenidiacea and Cladobranchia. (Wikipedia)

Doridoxidae, commonly known as doridoxid nudibranchs, is the only family in the superfamily Doridoxoidea, within the clade Pseudoeuctenidiacea.

Aeolidida

The Aeolidida is a taxonomic clade of sea slugs, specifically aeolid nudibranchs, marine gastropod molluscs in the clade Cladobranchia. They are distinguished from other nudibranchs by their possession of cerata containing cnidosacs.

Superfamily Flabellinoidea: Familes Flabellinidae, Notaeolidiidae.
Superfamily Fionoidea: Families Fionidae, Calmidae, Eubranchidae, Pseudovermidae, Tergipedidae.
Superfamily Aeolidioidea: Families Aeolidiidae, Facelinidae, Glaucidae, Piseinotecidae.
(Wikipedia)

Family Aeolidiidae

Flabellinidae is a taxonomic family of brightly coloured sea slugs, specifically nudibranchs, marine gastropod mollusks. (Clade Dexiarchia→Cladobranchia→Aeolidida→Superfamily Flabellinoidea→Family Flabellinidae)
Genera within the family Flabellinidae include Babakina, Calmella, Chlamylla, Cumanotus, Flabellina, ularia.

Left: Babakina indopacifica
Middle Left: Calmella cavolini The jaws are visible by transparency between the rhinophores.
Middle Right: Flabellina affinis Gulen Dive resort, Norway.
Right: Flabellina babai Banyuls-sur-Mer, France

Flabellina capensis (white edged nudibranch)

Left: Flabellina affinis Banyuls-sur-Mer, France.
Middle Left: Flabellina capensis (white edged nudibranch) Smitswinkel Bay wrecks, Cape Peninsula.
Middle Right: Flabellina browni Gulen Dive resort, Norway.
Right: Flabellina trophina

Spanish Shawl Nudibranch (Flabellina iodinea)

The Spanish shawl nudibranch (Flabellina iodinea)

Left: Spanish Shawl Nudibranch (Flabellina iodinea) The bright colors warn predators of its bad taste,. It can swim by making a series of quick lateral, U-shaped bends to its body, first to one side, then to the other, as seen here
Middle Left: The Spanish shawl (Flabellina iodinea) It is a species of Aeolid nudibranch, a very colorful sea slug.
Middle Right: The Spanish shawl nudibranch (Flabellina iodinea) has a dark purple foot and body, and is topped with orange cerata.
Right: Spanish shawl nudibranch (Flabellina iodinea) Channel Islands, California.

Family Facelinidae

The Facelinidae are a taxonomic family of colorful sea slugs. These are specifically aeolid nudibranchs. They are marine gastropod molluscs.
This family consists of the following subfamilies (according to the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005): Facelininae, Babakininae, Crateninae, Favorininae, Herviellinae, Pteraeolidiinae. (Wikipedia)

Hermissenda crassicornis Hermissenda crassicornis Hermissenda crassicornis
Hermissenda crassicornis (Clade Aeolidida, Superfamily: Aeolidioidea, Family: Facelinidae, Genus: Hermissenda)
Hermissenda crassicornis is a species of brightly coloured, sea slug or nudibranch, a marine gastropod mollusc in the family Facelinidae.
The species grows to be about 50 mm, or about 2 inches. The colour of this nudibranch varies from one locality to another, but it is always easily recognizable by the orange stripe along its head area and the white stripes on the cerata. (Wikipedia)

Left: Hermissenda crassicornis
Middle: Hermissenda crassicornis in La Jolla, California
Right:Hermissenda crassicornis about 2 inches long.

Left: Dondice banyulensis
Middle: Dondice banyulensis
Right:Dondice occidentalis