10 



COMPARATIVE ANATOMY 



CHAP. 



Order 3. OpisthobrancMa. 



The pleuro-visoeral connectives do not cross.' There is one auricle placed behind 

 the ventricle A shell is sometimes present, more freciuently 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- 

 plirodites with ovotestes. JIarine. 



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 

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

 podia, and mantle lobes covering the shell. 



A. Reptantia. 

 -(. Cephalaspidffi.— With frontal or cephalic disc. Fam. Actaeonidse (with 

 operculum), Scaphandridse, BuUidK (Bulla, Accra), GaBtropteridae (Fig. 14), 

 Philinidse, Doridiidse. 



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

 Fam. Aplysiidae (A'phjsia, Dolabella, Xutaixlnis). 



,1- — 



O 



Fio. ifi.— Pleurobranoliiis aurantiaous, witli internal 

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

 side, a, Rhinophores ; 6, labial sail ; c, genital aperture ; 

 d, nephridial apei-ture (?) ; c, ctenidium ; /, anus. 



•3 

 </ 



Fig. 14. — Gastropteron Meokelil, 

 with internal shell (after Vayssi^re). 

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

 podium ; 3, ctenidium, left almost un- 

 covered by ttie rudimentary mantle fold ; 

 4, flagelluiii = appendage of tlie mantle 

 fold. 



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

 (notreum) in or on which a shell may lie. Fam. Pleurobranchidse (Pleurobra-iichus 

 [Fig. 15], Pkurohrandiira, Oscainius), UmbrellidaB {UmlreUa, Tylodina), Peltidse. 



B. Natantia sive Pteropoda.^ 



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

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

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



"' Except in Actccaa, which is streptoneurous, and thus forms a connecting link 

 between tlie Opisthohranchia and Pnliiionata on the one liaud, and tlie remaining 

 Gastropods on the other [Bouvier and Pelseneer], v. Xnt. Sci., July 1893. 



^ The classification of the Opisthobranchs, which places tlie Pteropoda thecosomata 

 with the Ceplialaspidas, and the Pteropoda gymnosomata with the Anaspidfe, is accepted 

 on p. 110 and elsewhere. 



