312 



MANUAL OP THE MOLLTJSCA. 



Philine aperta tliere is no central tooth ; and the laterals, which 

 increase rapidly in sis^e backwards, have a finely denticulated 

 membranous inner edge. 



In Tornatella and Bulla (physis) the rachis is unarmed, and 



the lateral teeth are nume- 



/2=^ 



Fig. 136. Philine aperta. (Wilton.) 



reus and similar ; in Acera, 

 Cyliclini, and AmjpMsphyra 

 there is a minute central 

 tooth. 



The alimentary canal ter- 

 minates more in the rear of 

 the body than in the other 

 univalve shell-fish.* The 

 gills are behind the heart, 

 and the auricle behind the 

 ventricle ; conditions which 

 characterise the embryonic 



state of the moUusca generally. 



Comparatively little is known of the geographical distribution 

 of these animals ; they have been found wherever the requisite 

 search has been made, and are probably much more numerous 

 than at present estimated. Considerable additions, however, 

 have been made to our knowledge on this subject by the 

 researches of Kelaart in Ceylon and A. Adams in the Chinese 

 seas. The shell-bearing genera flourished in the period when 

 the secondary strata were deposited. The living species are 

 chiefly animal-feeders, preying on other shell-fish and on 

 zoophytes. 



Sectio]!^ a. — Tecti-brajstchiata.-j* 



Animal usually provided with a shell, both in the larval and 

 adult state ; branchiae covered by the shell or mantle ; sexes 

 united. 



FAMIIiY I. — TORlJiTATELLID^. 



Shell external, solid, spiral or convoluted ; sub-cylindrical ; 



» In the cuttle-fishes and pteropods it is bent upon itself ventrally, in the sea-snails 

 dorsally, terminating in front, near its origin ; the vascular system partakes of this 

 flexure, and the gills are in advance of the heart. (Huxley.) 



t Mono-pleuro-branchiata. Bl. Pomato-branchia, (from poma, a lid). Wiegm. 

 The order Tecti-branchiata of Cuvier included only the family JBullidce ; it is here 

 made to comprise the Infero-branchs also ; no object being gained by the multiplica- 

 tion of descriptive epithets. 



