OPISTHOBRANCHIA.] MOLLUSCA 655 Branch I. EUTHYNEURA (Spengel, 1881). Characters. Gastropoda Anisopleura in which the visceral loop (the conterminous visceral nerves) does not share in the torsion of the visceral hump, but, being placed entirely below the intestine, remains straight and untwisted, the junction of the visceral cords being below, and not I FIG. 52.BuUa vexilhim (Chemnitz), as seen crawling, ri, oral hood (compare with Tethys, fig. 02, B), possibly a continuation of the epipodia ; b, 6 , cephalic tentacles. (From Owen.) above, the intestine as it is in Streptoneura. Although the anus is not brought so far forward by the visceral torsion as in the Streptoneura, and may even by secondary growth assume a posterior median position, yet, as fully developed, an asymmetry has resulted as in the Azygo branchia, only the original right renal organ, right cteni- clium (if any), right osphradium, right side of the heart, and right genital ducts being retained. All the Euthy- neura are hermaphrodite. The lingual ribbon has very usually numerous fine denticles undifferentiated into series in each row. The shell is light and little calcified ; often it is not developed in the adult, though present in the embryo. An operculum, often found in d the embryo, is never present in FIG. 53. Tornatella. ft, shell ; b, , i i i j. / , m iii oral hood ; d, foot ; /, operculum. the adult (except in Tornatella, fig. 53). Many Euthyneura show a tendency to, or a complete accomplishment of, the suppression of the mantle- skirt as well as of the shell, also of the ctenidium, and ac quire at the same time a more or less cylindrical (slug-like) form of body. The Euthyneura comprise two orders, the Opistho- branchia and the Pulmonata. Order 1. Opisthobranchia. Marine Euthyneura the more archaic forms of which have a relatively large foot and a small visceral hump, from the base of which projects on the right side a short mantle-skirt. The anus is placed in such forms far back FIG. 54. Umbrella mediterranea. a, mouth ; b, cephalic tentacle ; 7i, gill (ctenidium). The free edge of the mantle is seen just below the margin of the shell (compare with Aplysia, fig. 63). (From Owen.) beyond the mantle-skirt. In front of the anus, and only partially covered by the mantle-skirt, is the ctenidium with its free end turned backwards. The heart lies in front of, instead of to the side of, the attachment of the ctenidium, hence Opisthobranchia as opposed to "Prosobranchia," Fio. 55. Umbrella mediterranea, seen from above, h, mouth ; i, cephalic tentacles ; k, penis-sheath. (After Keferstein.) which correspond to the Streptoneura. A shell is possessed in the adult state by but few Opisthobranchia, but all pass through a veliger larval stage with a nautiloid shell (fig. 60). Many Opisthobranchia have by a process of atrophy lost the typical ctenidium and the mantle-skirt, and have deve loped other organs in their place. As in some Azygo- branchia, the free margin of the mantle-skirt is frequently reflected over the shell when a shell exists ; and, as in some Azygobranchia, broad lateral outgrowths of the foot (epi podia) are often developed, which, as does not occur in Azy gobranchia, may be thrown over the shell or naked dorsal surface of the body. The variety of special deve lopments of structure accom panying the atrophy of typical organs in the Opisthobranchia and general degeneration of organization is very great, and renders their classification difficult. Two sections of the order may be distinguished, according as the typical Molluscan mantle-skirt (limbus pallialis) is or is not atro phied, and within each section certain sub-orders. Section a. PALLIATA (= Tectibranchiata, Woodward) the typical Molluscan mantle-skirt or pallium retained. Sub-order 1. Ctenidiobranchia. Characters. Palliata in which tlie ctenidium is retained as the branchial organ ; with rare exceptions a delicate shell, which may be very small or completely enclosed by the reflected margin of the mantle; epipodia (lateral outgrowths of the foot) frequently present. Family 1. Tornatcllidfe. Genera : Tornatella, Lam. (fig. 53) ; Cinulia, Gray, &c. Family 2. Bullidie. Genera : Bullet, Lam. (fig. 52) ; Accra, Miiller ; Scaphander, Montf. ; Bullsea, Lam. ; Doridium, Meckel ; Gastroptcron, Meckel, &c. Family 3. Aplysiidse. Genera: Aplysia, Gmelin (the Sea-Hare) (figs. 20, 56, &c.) ; Dolobclla, Lam.; Lobiger, Krohn, &c. Family 4. Pleurobranchidse. Genera: Pleurobranchus, Cuvier; Umbrella, Chemnitz (figs. 54, 55); Runcina, Forbes, &c. Sub-order 2. Phyllidiobranchia. Characters. Palliata in which the ctenidia have atrophied ; much as in PatellidiE among the Zygobranchiate Streptoneura their place is taken by laterally-placed lamella;, developed from the inner surface of the bilaterally-disposed mantle-skirt in two lateral rows. Family 5. Phyllidiadse. Genera : Phyllidia, Cuiver ; Pleuropliyllidia, Meek. (fig. 57). Section b.NON-PALLIATA. CJiaractcrs. The typical Molluscan mantle-skirt is atrophied in the adult. No shell is present in the adult, though the dorsal integument may be strengthened by calcareous spicules (Doris). The otocysts are not sessile on the pedal ganglia as in otber Gastropods, but, as in the Natantia Azygobranchia, lie close to the cerebral ganglia. In one sub-order (Pygobranchia) the typical ctenidium appears to be retained in a modified form ; in the others special developments of the body-wall take its place, or no special respiratory processes exist at all. The general form of the body is slug-like, the foot and visceral hump being coextensive, and a secondary bilateral symmetry is asserted by the usually median (sometimes right-sided) dorsal position of the anus on the hinder part of the body. Sub-order 1. Pygobranchia. Characters. The ctenidium assumes the form of a circlet of pinnate processes surrounding the median dorsal anus ; a strongly-marked epipodial fold may occur all round the foot and simulate a mantle- skirt (see fig. 62, C, Doris) ; papilla; or " cerata " of the dorsal integu ment may occur as well as the true ctenidium (fig. 61). Family 6. Dorididse. Genera: Doris, L. ; Goniodoris, Forbes; Triopa, Johnst. ; ^Egirus Loven ; Thccarera, Fleming ; Polijccra, Cuvier ; Idalia, Leuck-
art ; Ancv.la, Loven; Ccratosoma, Adams; Onchidoris, Blainv.