10 



COMPARATIVE ANATOMY 



CHAP. 



ORDER 3. Opisthobranchia. 



The pleuro- visceral connectives do not cross. 1 There is one auricle placed behind 

 the ventricle. A shell is sometimes present, more frequently wanting. An 

 operculum is rarely found. Respiration by means of true ctenidia, or of adaptive 

 (ills, or through the skin. The visceral dome is very often levelled down. Herma- 

 phrodites with ovotestes. Marine. 



Sub-Order 1. Tectibranchia. 



The pallial complex is to the right of the body, and is more or less covered by 

 the mantle fold belonging to that side. One true ctenidium (viz. that which was 

 originally the right) is always retained in the mantle cavity, but is often very 

 incompletely covered by the mantle. The visceral dome tends to disappear. A 

 shell is always present, but tends to become rudimentary. Generally with para- 

 podia, and mantle lobes covering the shell. 



A. Reptantia. 



frontal or cephalic 



disc. Fam. Actseonidse 

 Accra], Gastropteridse 



(Fig. 



(with 



14), 



a. Cephalaspidse. "With Irontal or 

 operculunO, Scaphandridae, Bullidse (Bulla, 

 Philinidae, Doridiidse. 



b. Anaspidae. Head without frontal disc ; four triangular or ear-like tentacles. 

 Fam. Aplysiidse (Aplysia, Dolabclla, Notarchus). 







FIG. 14. Gastropteron Meckelii, 

 with internal shell (after Vayssiere). 

 1, Cephalic shield (frontal disc) ; 2, para- 

 podium ; 3, ctenidium, left almost un- 

 covered by the rudimentary mantle fold ; 

 4, flagellum= appendage of the mantle 

 fold. 



FIG. 15. Pleurobranchus aurantiacus, with internal 

 shell (after Leuckart's Wamltcifdn), seen from the right 

 side, a, Rhinophores ; &, labial sail; c, genital aperture ; 

 /. nrphridial aperture (?) ; c, ctenidium ; /, anus. 



c. Notaspidae. Head short, with or without tentacles. Large dorsal disc 

 (notfeum) in or on which a shell may lie. Fam. Pleurobranchidse (Pleurobranchus 

 [Fig. 15], Plcurobrancluca, Oscainius), Umbrellidae ( Umbrella, Tylodina), Peltidse. 



B. Natantia sive Pteropoda.- 



These formerly constituted a separate class of the Molluscs, but are now recog- 

 nised to be Tectibranchia adapted to a free-swimming pelagic life. The parapodia of 

 the Tectibranchia develop as fins or wing-like swimming organs. 



1 Except in Actun/i, which is streptoueurous, and thus forms a connecting link 

 between the Opisthobranchia and Pulmonata on the one hand, and the remaining 

 Gastropods on the other [Bouvier and Pelseneer], r. Ifat. Sci., July 1893. 



J The classification of the Opisthobranchs, which places the Pteropoda thecosomata 

 with the Cephalaspidre, and the Pteropoda gymnosomata with the Anaspidse, is accepted 

 on p. 110 and elsewhere. 



