118 



MOLLUSCA 



Branch h.—EUTH7NEURA (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 sunk 

 entirely below the body-wall, remains straight and un- 

 twisted. Although the anus is not brought so far forward 



ribbon has very 

 I 



Fig. 52. — Bulla vexiltum (Chemnitz), as seen crawling, d, oral hood (compare 

 with Tethys, flg. 62, B), possibly a continuation of the epipodla; 6, t', cephalic 

 tentacles. (From Owen.) 



by the visceral torsion as in the Streptoneura, and may even 



hy secondary growth assume a posterior median position, yet, 



as fully developed, an asymmetry has resulted as in the 



Azygobranchia, only the original right renal organ, right 



ctenidiuro (if any), right osphradium, right side of the heart, 



and right genital ducts being retained. AU the Euthy 



neura are hermaphrodite. The lingual 



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 



the embryo, is never present in Pic 53.— Tomateiia. ft, shell ; 6, 



the adult (except in TornateUa, "'^i hood ; d, foot ;/, operculum. 



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 ig placed in such forms far back 



Fig. 54. — Umbrella Tnediterranea. a, mouth ; 6, cephalic tentacle ; A, gill 

 (ctenidium). The tree edge of the mantle is seen just below the margin of 

 the sheU (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, 

 -whence Opisthobranchia as opposed to " Prosobranchia," 



■ Fig. 55. — Umbrella Tnsditerraneat seen 

 from above, h, mouth ; i, cephalic 

 tentacles; Tc, penis-sheath. (After 

 Eeferstein.) 



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 th( 

 mantle-skirt, and have deve 

 loped other organs in theii 

 place. As in some Azygo 

 branchia, the free margin ot 

 the mantle-skirt is frequently 

 reflected over the shell whei 

 a shell exists; and, as in som( 

 Azygobranchia, broad lateral 

 outgrowths of the foot (epi- 

 podia) are often developed, 

 which, a,s 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 

 MoUuscan mantle-skirt (hmbus pallialis) is or is not atro- 

 phied, and within each section certain sub-orders. 



Section a. — PALLIATA { = Tedibranchiata, 'Wood-Hfard) — the 

 typical MoUuscan mantle-skirt or pallium retained. 

 Sub-order 1. — CtenidiobrancTiia. 

 Characters. — Palliata in which the ctenidium is retained as the 

 hranohial organ ; -with rare exceptions a delicate shell, -which may- 

 be .very smafl or completely enclosed by the reflected margin of the 

 mantle; epipodia (lateral outgro-wths of the foot) fiequently present. 

 Family 1. — TomaUllidse. 



Genera : Tomateiia, Lam. (fig. 53) ; Cinulia, Gray, &c. 

 Family 2. — Bullidas. 

 Genera : Bulla, Lam. (fig. 52) ; Acera, Miiller ; Scaphander, 

 Montf. ; Bullsea, Lam. ; Doridmm, Meckel ; Gastrqpteron, 

 Meckel, &c. 

 Family 3. — Aplysiidse. 



Genera: Aplysia, Gmelin (the Sea-Hare) (figs, 20, 56, &c.) ; 

 Dolobella, Lam.; Lobiger, Krohn, &c. 

 Family i. — Pleurobranchidas. 

 Genera : Pleurobranchus, Cu-vier ; Umbrella, Chemnitz (figs. 54, 

 65); Mioncma, Forbes, &c. 



Sub-order 2. — Phyllidiobranchia. 



Characters. — Palliata in -which the ctenidia have atrophied ; much 

 as in Patellidae among the Zygobranchiate Streptoneura their place 

 is taken by laterally -placed lamellae, developed from the inner surface 

 of the bUateraUy-disposed mantle-skirt in two lateral rows. 

 Family 5. — Phyllidmdas. 



Genera : Phyllidia, Cuiver ; Pleurophyllidia, Meek. (fig. 57). 



Section h.—NON-PALLIATA. 

 Characters. — The typical MoUuscan mantle-skirt is atrophied in 

 the adult. No sheU is present in the adult, though the dorsal 

 integument may be strengthened by calcareous spicules (Doris) . The 

 otocysts are not sessUe on the pedal ganglia as in other 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 bUateral 

 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. — Fygob/ramchia. 

 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, 0, Doris) ; papiUse or " cerata " of the dorsal integu- 

 ment may occur as weU as the true ctenidium (fig. 61). 

 Family 6. — Dorididas. 



Genera : Doris, L. ; Goniodoris, Forbes ; Triopa, Johnst. ; ^girus, 

 Loven ; Thecacera, Fleming ; Polycera, Cuvier ; Idalia, Leuck- 

 art ; Ancula, Loven ; Ceratosoma, Adams ; Onchidoris, Blainv. 



