338 



COMPARATIVE ANATOMY. 



distinctly allied to the arrangement in the Zeugobranchia, is adapted 

 to the asymmetry of the branchial cavity, which, again, is dependent 

 on the characters of the shell. The smaller gill is generally approxi- 

 mated to the other, and becomes asymmetrical in position ; in 

 some Prosobranchiata it disappears altogether (Janthina, Neritaceas, 

 Heteropoda). 



The right gill is generally developed on one side only, so that 

 it is semi-pinnate, owing to the disappearance of the second row of 

 lamellae. Although, as a general rule, we find the lamellar structure 

 to be the most common, a few (Calyptra3a, Crepidula) have fila- 

 mentous gills, and so call to mind the primitive form of the Lamelli- 

 branch gill. 



The gills become modified, and may disappear altogether when 

 the mantle and the branchial cavity are atrophied. This happens 

 in various divisions ; thus, in the Heteropoda, among the Proso- 

 branchiata, the gill of Carinaria is not covered over by the mantle ; 

 in Pterotrachea, where there is no mantle at all, the gill is quite 

 free, while in Firoloi'des, the gill, as well as the mantle, disappears. 



Among the Opisthobranchiata the characters of the gills are 

 equally dependent on the condition of the mantle. There is a gill 

 on either side, between the mantle and the foot (Pleurophyllidia), or 

 there is only a single gill in the gill-chamber, or, finally, the gill is 

 only partly covered over by the mantle (Tectibranchiata) . When 



the shell and mantle disappear, gill-like struc- 

 tures may be developed on the dorsal surface of 

 the body, as in some of the Nudibranchiata. 



Lamellar, or tufted and branched appen- 

 dages, are sometimes developed in the anal 

 region (Doris), sometimes in rows over the 

 whole body (Tritonia, Scyllasa). If we are 

 right in regarding the possession of a shell by 

 the larvae of all the Opisthobranchiata as a fact, 

 which proves conclusively that these Gastro- 

 poda are derived from shelled forms, and if wo 

 must suppose that the primitive position of tlio 

 gills was within the mantle- cavity, then we 

 must regard the arrangement of the gills in 

 the Dorididae as having been inherited in its 

 essential features from this condition, for we 

 must remember that the anus also is placed in 

 the mantle -cavity. There are many inter- 

 mediate steps between this and the more 

 general distribution of gills over the back of 

 the body ; and further, these gills, howsoever 

 modified, and howsoever peculiar their form, 

 are never anything more than mere processes 

 of the integument. Their varied external form 

 is due to their superficial position, which is due to the loss of the 

 mantle which invested them ; and it is because of its absence that they 



Tig. 177. Ancula 

 (Polycera) cristata; 

 dorsal view. a Anus. 

 Ir Branchia?. t Ten- 

 tacles (after Alder and 

 Hancock). 



