THE MOLLUSCA. 405 



A systemic heart usually exists, and when present is situ- 

 ated in the middle of the posterior haemal region, and consists 

 of, at fewest, two chambers, an auricle and a ventricle. Arte- 

 rial vessels often ramify extensively through the body, but 

 more or fewer of the venous channels remain in the condition 

 of lacunae. The blood-corpuscles are colorless and nucleated. 

 Distinct respiratory organs may be absent, or they may take 

 the form of branchiae or pulmonary sacs. When present, they 

 lie in the course of the blood which is returning to the heart. 

 Beside the heart and the intestine are situated the renal or- 

 gans, which, on the one side, open externally, and on the 

 other communicate with the blood system. 



The nervous system consists of, at least, one pair of ganglia 

 (cerebral) at the sides, or on the haemal aspect of, the mouth, 

 and of two other pairs of oesophageal ganglia (pedal and 

 parieto-splanchnic). The latter are situated at the sides, or 

 on the neural aspect, of the alimentary canal, and are con- 

 nected by commissures with the former. 



In the majority of the Mollusca, the embryo passes through 

 a stage in which it is provided with bands of cilia or with a 

 simple, bifid, or multind fold of the integument (velum), the 

 edges of which are ciliated, developed on the haemal aspect of 

 the cephalic region of the body, in front of the pallial region. 



The special peculiarities of the different groups of the 

 Mollusca result chiefly — 



1. From the form of the pallial region, and the extent of 



the mantel-lobes relatively to the body. 



2. From the number and arrangement of the pieces of the 



shell to which the mantle gives rise. 



3. From the proportional size and the form of the foot 



and the production, or non-production, of chitinous, 

 or shelly, matter by it. 



4. From the development of sense-organs on the anterior 



end of the body, and the absence or presence of a 

 distinguishable head. 



5. From the disproportionate growth of the haemal region 



of the body into a visceral sac, followed by a change 

 in the primitive direction of the intestine, and often 

 accompanied by asymmetrical lateral distortion. 



The Lamellibranchiata. 1 — In these Mollusks there are 



1 For a description of the anatomy of a Lamellibranch in detail, the student 

 is referred to Huxley and Martin,' " Elementary Biology," and Rolleston, 

 " Forms of Animal Life." 



