THE MOLLUSCA 
Los) 
Ww 
Lamellibranchs ; twenty-four hours in Pholas, ete. The eggs of 
Gleba, aggregated into a nidus, are hatched after three or four 
days, those of Ischnochiton after seven days ; but in the majority of 
Gastropods and the Cephalopods the time required is much longer. 
The marine larvae of temperate seas are intolerant of a rise of 
temperature, and generally perish when it approaches 30° C. 
IV. DEFINITION OF THE MOLLUSCA. 
From what precedes, it results that in each of the five classes 
the same lines of specialisation may be observed—viz. the loss of 
the shell, of the foot, of the ctenidia, of the radula, ete. Thus the 
general morphological characters are obviously those of the most 
primitive of the different classes (Fig. 19), and the following 
diagnosis may be proposed for the phylum :— 
1. The Mollusca are originally bilateral organisms, in which 
signs of primitive segmentation are no longer evident. 
Fic. 19. 
Scheme of a primitive Mollusc, viewed from the left side. a, anus ; c.g, cerebral ganglion ; 
f, foot ; g, gill, in the pallial cavity ; go, gonad; h, heart; k, kidney ; la.c, labial commissure ; 
m, mouth; pa, mantle; pa.n, pallial nerve; pe, pericardiuin ; p.g, pedal ganglion; pl.g, 
pleural ganglion ; ra, radula; r.p.o, reno-pericardial orifice ; st, stomach ; st.g, stomato-gastric 
ganglion ; v.g, visceral ganglion. 
2. They possess a well-developed coelom (gonad and _pericar- 
dium), enteron, and haemocoel, quite distinct from one another. 
3. The alimentary tract exhibits (or has lost) a radular sac in 
its anterior part. 
4. The nervous system consists of a peri-oesophageal ring, 
whose supra-oesophageal (or dorsal) moiety is the cerebral com- 
missure, and the infra-oesophageal (or ventral) moiety is the labial 
commissure. The former gives off chiefly sensorial nerves, the 
latter nerves to the digestive tract. From their union two nervous 
cords arise on each side, a dorsal or pallial and a ventral or pedal ; 
from the former arise the visceral nerves, whose main trunks are 
frequently joined together under the digestive canal to form the 
infra-intestinal visceral commissure. 
5. The general body-wall is differentiated into three regions: 
