THE MOLLUSCA. 405 



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

 ated in the middle of tli(^ posterior ha-mal r('*;'ion,and consists 

 of, at fewest, two chambers, an aurich.^ 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 lacuiue. The bloo(l-cori)uscles are coloi-less and nueleated. 

 JJislinct res})iratory organs may be absent, or they may take 

 the form of brancliiie or pulmonary sacs. When ])resent, they 

 lie in tlu^ course of the blood whieli is rcluriiing to the heart. 

 Beside the heart ami tlui intestine aie situated the renal or- 

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

 other comnnmicate with the blood system. 



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

 {cerebral) at the sides, or on the haemal asi)ect of, the mouth, 

 and of two other pairs of a'sophageal ganglia {pedal and 

 parleto-s])lanch)ilc). The latter are situated at the sides, or 

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

 nected by commissure's with the former. 



In i\u) majority of the Mollasca, the embryo passes through 

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

 simple, bifid, or nniltilid fold of the intcgunK'ut {velw/i), the 

 edges of whieh are ciliated, develojied on the luemal aspect of 

 the cephalic region of the body, in fi'ont of the [)allial region. 



The special peculiarities of the dillerent groups of the 

 MoUusca result chi(!fly — 



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



the mantel-lobes relatively to the body. 



2. From tlie 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 develojimcnt of sense-organs on the anterior 



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

 distinguishable head. 



5. From the dis|)roportionate growth of the hannal region 



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

 in the [)rimitivo direction of the intestine, and often 

 accompanied by asymmetrical lateral distortion. 



The Lamkllibka-NCiiiata.* — In these MoUusks there are 



» For a dccription of thn anatoiny of a Tjam((nil)ranoli in clotail, tlic student 

 is rcfbrrefl to IIuxl(3V and Murtinj '■' Elomentury Biology," and Uolkston, 

 " Forms of Aninaal Life." 



