MOLLUSCOIDA. 



195 



The mouth is placed between the bases of the arms, and is not 

 furnished with any apparatus of teeth. It con- 

 ducts by a gullet into a distinct stomach, sur- 

 rounded by a well-developed granular liver. The 

 intestine may or may not be furnished with a 

 distinct anus, but in no case does it open into 

 the body-cavity. Within the lobes of the mantle 

 there is a remarkable system of branched tubes, 

 which commence by blind extremities, and finally 

 communicate with the mantle - cavity by means 

 of certain organs which were formerly believed 

 to be hearts, and are now known as " pseudo- 

 hearts." This system of tubes appears to be main- 

 ly, if not entirely, connected with reproduction. A 

 true heart, however, is present in moat, if not in all, 

 of the Brachiopoda. 



The nervous system consists of a single principal 

 ganglion, in some cases coimected with others so 

 as to form a collar round the commencement of the 

 gullet. In some cases, however, the nervous system 

 appears to be very rudimentary. 



The sexes appear to be sometimes distinct and 

 sometimes united in the same individual. The 

 embryo, in some cases, at any rate, is locomotive, 

 moving from place to place by means of the ciliated 

 arms or by ventral spines. 



Fig. 136. — JAngula 

 atiatina, showing 

 the muscular stalk 

 hywhich the shell 

 is attached (p ), 

 (After Cuvier). 



