312 MANUAL OF THE MOLLUSCA. 
Philine aperta there is no central tooth; and the laterals, which 
increase rapidly in size backwards, haye a finely denticulated 
membranous inner edge. 
In Tornatella and Bulla (physis) the rachis is unarmed, and 
the lateral teeth are nume- 
EN aa rous and similar; in Acera, 
{ SENS Cylichna, and Amphisphyra 
Ti there 1s a minute central 
L\ tooth. 
a The alimentary canal ter- 
Z \\ minates more in the rear of 
i}\\ the body than in the other 
a, /} univalve shell-fish.* The 
gills are behind the heart, 
and the auricle behind the 
Fig. 186. Philine aperta. (Wilton.) eam conditions which 
cae ia: characterise the embryonic 
state of the mollusca 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. 
Section A.—TECTI-BRANCHIATA.+ 
Animal usually provided with a shell, both in the larval and 
adult state; branchizo coyered by the shell or mantle; sexes 
united. 
FAmiIty I.—ToRNATELLIDA. 
Shell external, solid, spiral or conyoluted; sub-cylindrical ; 
* In the cuttle-fishes and pteropods it is bent upon itself ventrally, in the sea-snails 
dorsally, terminating in front, near itg origin; the vascular system partakes of this 
fiexure, and the gills are in advance of the heart. (Huxley.) 
+ Mono-pleuro-branchiata. Bl. Pomato-branchia, (from poma, a lid). Wiegm. 
The order TYecti-branchiata of Cuvier included only the family Bullide ; it is here 
made to comprise the Jnfero-branchs also; no object being gained by the multiplicu. 
tion of descriptive epithets, 
