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 

 gills, 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. 



a. Cephalaspidse. With frontal or cephalic disc. Fam. Actseonidse (with 

 operculum), Scaphandridse, Bullidse (Sulla, Acera), Gastropteridae (Fig. 14), 

 Philinidae. Doridiidse. 



6. Anaspidse. Head without frontal disc ; four triangular or ear-like tentacles. 

 Fam. Aplysiidse (Ajjlysia, Dolabella, Notarchus). 



FIG. 14. Gastropteron Meckelii, FIG. 15. Pleurobranchus aurantiacus, with internal 



with internal shell (after Vayssiere). shell (after Leuckart'S Wandta.feln), seen from the right 



1, Cephalic shield (frontal disc) ; 2, para- side, ft, Rhinophores ; b, labial sail ; c, genital aperture ; 



podium; 3, ctenidium, left almost un- d, nephridial aperture (?); e, ctenidium ;/, anus, 

 covered by the rudimentary mantle fold ; 

 4, flagellum = appendage of the mantle 

 fold. 



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

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

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



B. Natantia sive Pteropoda. 2 



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 Actceon, 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], v. Nat. Sci., July 1893. 



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

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

 on p. 110 and elsewhere. 



