Section 2. Holohepatica.--Cerata Medio-Dorsal, Retractile or Not,

usually paucifoliate, liver never ramified, usually no jaws.

FAM. 1. Dorididae.--Branchia consisting of a circle or semicircle of pinnate leaves united at the base, surrounding the anus, almost always retractile into a cavity, rhinophores foliate, no suctorial proboscis, radula multiseriate. Genera: Bathydoris, Hexabranchus, Archidoris (Fig. 289), Discodoris, Diaulula, Cadlina, Centrodoris, Platydoris, Chromodoris, Miamira, with many sub-genera.

FAM. 2. Doriopsidae.--Branchia and rhinophores as in Dorididae, oral aperture pore-shaped, suctorial, no radula. Single genus, Doriopsis.

FAM. 3. Phyllidiidae.--Body oval, depressed, leathery, a ring of branchial lamellae, only interrupted by the head and genital papilla, under the pallial edge, oral aperture pore-shaped, suctorial, no radula. Genera: Phyllidia, Fryeria. Bergh unites this and the preceding family in the group Porostomata, which, with Fam. 1, form the group Dorididae cryptobranchiatae.

FAM. 4. Polyceridae.--Body slug-like, branchiae not retractile, usually surrounding the anus, rhinophores foliate, tentacles simple, radula variable, central tooth generally wanting. Genera: Notodoris, Triopella, Aegires, Triopa, Issa, Triopha, Crimora, Thecacera, Polycerella, Palio, Polycera, Ohola, Trevelyana, Nembrotha, Euplocamus, Plocamopherus, Kalinga.

FAM. 5. Goniodoridae.--Body oval, depressed, branchia multifoliate, usually disposed in shape of a horse-shoe, rhinophores foliate, retractile or not, mouth with a large suctorial proboscis, radula variable. Genera: Akiodoris, Doridunculus, Acanthodoris, Adalaria, Lamellidoris, Calycidoris, Goniodoris, Idalia, Ancula, Drepania.

FAM. 6. Corambidae.--Body otherwise Doris-like, but with two posterior branchiae under the mantle edge, jaws present, no central tooth, about five laterals. Single genus, Corambe (= Hypobranchiaea). Bergh unites this and the two preceding families in the group Dorididae phanerobranchiatae.

=Sub-order IV.= =Pteropoda.=--The Pteropoda are pelagic animals in which the lateral portions of the foot are modified into fins, which are innervated by the pedal ganglia. Their systematic position has undergone recent revision. It has been the custom to regard them as an Order of equivalent value to the other four, while some have held them to be a subdivision of Cephalopoda. Modern authorities, chief among whom is Pelseneer, regard the Pteropoda not as a primitive, but as a derived and recent group. They are “Gasteropoda in which the adaptation to pelagic life has so modified their external characters as to give them an apparent symmetry.”

The principal point which relates the Pteropoda to the Gasteropoda is the asymmetry of the visceral organs, intestine, heart, kidney, and genital gland, which results from their development on one side only of the body. Their hermaphroditism and the structure of their nervous system relate them to the Euthyneura rather than to the Streptoneura. Resemblances in the organs of circulation and generation approximate them to the Opisthobranchiata rather than to the Pulmonata, while of the two groups of the former, they tend to closer relationship with the Tectibranchiata than with the Nudibranchiata. The two sections of Pteropoda have been considered of distinct origin, the Thecosomata being derived from the Bulloidea, the Gymnosomata from the Aplysioidea.

Thus the Pteropoda are a group whose true relations are masked by the special conditions of their existence, which have tended towards the development of certain organs, the so-called “wings” and the shell, which give them an apparent symmetry; this symmetry disappears on a closer investigation of the internal organs. They are hermaphrodite; the genital gland has a single efferent duct (except in some Cavolinia), a seminal groove leading to the copulatory organ, which in the Thecosomata is on the right side of the head, in the Gymnosomata on the right side of the foot. The genital system resembles that of the Opisthobranchiata and of the “digonoporous” Pulmonata.