.'392 



MOLLUSC A CONSIDERED AS A BRANCH. 



F:G. 526. 



FIG. 527. 



Fig. 528. 



Magnified portion of 

 Pearl-shell . 



Magnified portion of 

 Pinna. 



Magnified portion of 

 Terebratula. 



not red. Their circulatory system consists of a heart, 

 arteries and veins, and venous sinuses. Their respiratory 

 organs are gills, or lung-like cavities. Their muscular 

 system is well developed. Their nervous system is mainly 



FIG. 529. 



'Nervous System in the Molhiaca. 



a, ft, c, d, nerve-ganglions. 



The principal center of the nervous system is in the form of a ring surrounding 

 the gullut. 



in the form of a ring which surrounds the oesophagus 

 or throat, from which nervous filaments radiate and con- 

 nect with other nervous ganglia in other parts of the body. 

 As to reproduction, most kinds of mollusks increase 

 by means of eggs. Some kinds, as the River-Snails 

 (Paludina), etc., are called viviparous, because they re- 

 tain the eggs in the oviduct till after they are hatched, 

 and the young have attained a considerable size. Some 

 kinds of tunicate mollusks increase by budding. 



