118 



MOLLUSCA 



Branch b. 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 suuk 

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

 twisted. Although the anus is not brought so far forward 



FIG. 52. Bulla texilhim (Chemnitz), as seen crawling, d, oral hood (compare 

 with Tethys, fig. 62, 13), possibly a continuation of the epipodia ; 6,6', cephalic 

 tentacles. (From Owen.) 



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 

 Azygobranchia, only the original right renal organ, right 

 ctenidiuro (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 

 rot developed in the adult, 



though present in the embryo. 



An operculum, often found in d 



the embryo, is never present in F>. 53. Tornateiia. K, shell ; 6, 

 the adult (except in Tornateiia, llhood : d ' foot : ' opercul 

 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 



Fio. 54. Umbrella mediterranea. a, mouth ; 6, cephalic tentacle ; h, 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," 



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 Flo ^_ nmbreUa 



surface of the body. f ro ' above, h, mouth 



rm . . f -11 tentacles ; k. 



The variety of special deve- Kcfcrstein.) 



t, cephalic 

 penis-sheath. (After 



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 mautle-skirt or pallium retained. 



Sub-order 1. Ctenidiobranchia. 



Characters. Palliata in which the 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; epi podia (lateral outgrowths of the foot) fi equently present. 

 Family 1. Tornatcllidse. 



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

 Family 2. Bullidse, 



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

 Montf. ; Bullsea, Lam. ; Doridium, Meckel ; Gastropteron, 

 Meckel, &c. 

 Family 3. Aplysiidse. 



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



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

 Family 4. Plcurobraiichidas. 



Genera: Pleurobranchus, Cuvier; Umbrella, Chemnitz (figs. 54, 

 55); Euncina, Forbes, &c. 



Sub-order 2. Phyllidiobranchia. 



Characters. Palliata in which the ctenidia have atrophied ; much 

 as in Patellid;e among the Zygobranchiate Streptoneura their place 

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

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

 Family 5. Phyllidiadas. 

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



Section &. NON-PALLIA TA. 



Characters. 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 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 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) ; papillae 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 ; TriojM, Johnst. ; dZgirus, 

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

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



