246 
POrULAR SCIENCE REVIEW. 
The Mantle . — This organ, upon whose presence the very 
existence of the shell depends, completely enshrouds the rest of 
the animal. It is made up of two equal lateral lobes, which 
are adherent along their dorsal edge — that nearest the hinge — 
being perfectly separate, or “ free,” along their anterior, pos- 
terior, and ventral edges. In bivalves provided with siphons* 
e.g., the “ gaper ” (Mya) and the “ piddock ” ( Pholas\ they are 
adherent along all their edges, with the exception only of a 
small extent of the ventral margin, where they are separate to 
allow of the protrusion of the foot. 
Into the space, or pallial chamber, included between the 
lobes of the mantle, project the foot, tentacles, and gills. 
Owing to the junction of the inner leaflets of the gill of either 
side, for about the extent of their posterior half, a kind of 
partition is formed, which fences off two chambers of unequal 
size. The larger and more ventrally placed may be termed 
the “ branchial” chamber ; its entrance is guarded by the 
fringed posterior edges of the mantle-lobes, which, therefore, 
may be considered as rudimentary homologues of the 66 inha- 
lant” siphon in My a, Pholas , and others. Into the smaller or 
dorsal chamber opens the anus ; its orifice, therefore, which 
transmits faecal matter, and water which has been robbed 
by the gills of its dissolved air, corresponds to that of the 
“ exhalant” siphon in the bivalves just mentioned. This cham- 
ber may be conveniently called 66 anal.” 
The inner surface of the mantle, or that which looks into the 
pallial chamber, is lined with ciliated epithelial cells, while 
the cells which cover the outer surface, or that which secretes 
the nacre and the prismatic layer of the shell, are of the non- 
ciliated variety. 
Digestive System . — The mouth (fig. 1, m), which is not pro- 
vided with any organ for the mastication of food, is a somewhat 
triangular opening situated in advance of the foot, and just 
behind the anterior adductor muscle. Its lips are guarded by 
a pair of leaf-shaped appendages, the labial tentacles (fig. 1, 
rt 9 1 1 ), each made up of two leaflets, joined together at their 
dorsal edges, and having their inner, or apposed, surfaces marked 
for the greater part of their length with transverse ridges. 
The inner leaflets by their junction form the posterior, and the 
outer, the anterior boundary of the mouth. The gullet, the 
lining membrane of which is marked by longitudinal ridges* 
leads soon into a stomach, somewhat irregular in shape, and 
communicating on its right side with a blind sac (figs. 2 
and 3, s £), or “ caecum,” which lodges a peculiar body — 
the “ crystalline style ” (“ Krystallstiel,” Meckel). This organ* 
the nature and function of which is unknown, varies very much* 
but is “ found,” according to Professor Rolleston, “ in greatest 
