Section Iv. Siphonarioidea.--Shell Patelliform, Branchia Replaced

wholly or in part by a pulmonary sac, pulmonary orifice closed by a small lobe, radula multiseriate, teeth very small.

FAM. Siphonariidae.--Characters those of the section. Genera: Siphonaria (branchia as well as pulmonary sac), Gadinia (no branchia). These genera, hitherto placed among the Pulmonata, have been recently shown (see p. 19) to be modified Opisthobranchiata.

=Sub-order II.= =Ascoglossa.=--Branchia, mantle cavity, and shell generally wanting, liver ramified, rami enclosed in external papillae (cerata) or beneath the dorsal surface, kidney not compact, branched; radula with one series of strong teeth (Fig. 288), worn out teeth at the front end not dropping off, but preserved in a special sac (ἀσκός).

According to Bergh, the Ascoglossa form a link between the Tectibranchiata,--especially the Aplysiidae and Bullidae--and the Cladohepatic Nudibranchs, while the Pleurobranchidae form a somewhat similar link between the Holohepatic Nudibranchs and the other Tectibranchiata.

FAM. 1. Oxynoeidae.--Animal long, tentacles auriform, epipodia large, simple, or wing-like, a ctenidium and branchial chamber on right side, shell small, thin, slightly spiral, not covering much of the body. Genera: Oxynoe (= Lophocercus), Lobiger.

FAM. 2. Hermaeidae.--Body depressed, cerata in several rows, no branchiae, no shell. Genera: Hermaea, Phyllobranchus, Stiliger, Alderia.

FAM. 3. Elysiidae.--Body depressed, head rather elevated, tentacles auriform, sides of body dilated into two large wings, which enclose branches of the liver and sometimes fold over the dorsal surface, no branchiae, no shell. Genera: Elysia, Thridachia, Placobranchus.

FAM. 4. Limapontiidae.--Body slug-like, liver scarcely ramified, no branchiae, shell, or appendages. Genera: Limapontia, Actaeonia, Cenia.

=Sub-order III.= =Nudibranchiata.=--Shell absent in the adult, no ctenidium proper, or osphradium, cerata dorsal or dorso-lateral, nervous system concentrated, kidney not compact, ramified, penis retractile, jaws and radula usually present.