32 THE MOLLUSC A 



Lamellibranchs ; twenty-four hours in PJtolas y etc. 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 )F 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, etc. 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. 



Fio. 19. 



Scheme of a primitive Mollusc, viewed from the left side, a, anus ; c.g, cerebral ganglion ; 

 /, foot ; g; gill, in the pallial cavity ; 0o, .gonad ; h, heart ; k, kidney; la.c, labial commissure ; 

 m, mouth ; pa, mantle ; pa.n, pallial nerve ; pe, pericardium ; p.g, pedal ganglion ; pl.g, 

 pleural ganglion ; ra, radula ; r.p.o, reno-pericardial orifice ; st, stomach ; st.gr, 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 : 



