340 



INVEETEBEATA 



CHAP. 



a post-oral tuft of cilia, the sole representative of the metatroch of 

 Annelida. The oesophagus is ciliated, and small organisms are whisked 

 into the stomach. The modification of the anterior wall of the stomach 

 into the liver-pouches has already been mentioned. From its posterior 

 ventral wall a short pouch grows out on the left side, whose cells 

 secrete rod-like excrescences. This is the rudiment of the crystal- 

 line sac which secretes the crystalline style (Figs. 269, 270). 



A larval kidney, consisting of a straight tube opening at the 



cr.s 



, Fig. 270. — Young Veliger larva of Dreissensia polymorpha, seen from the ventral surface. 



(After Meisenheinier.) 



Letters as in previous figure. In addition, m.c, mantle-eavity. 



side of what is afterwards the foot, and terminating internally in a 

 solenocyte situated near the liver-pouch, makes its appearance at 

 the same time as the shell and disappears as the foot grows out. 

 Meisenheimer derives it from an ingrowth of the ectoderm, but he 

 has no convincing evidence to prove this. We think it more probable 

 that it arises, as Wierzejski (1905) has proved that it arises in Physa, 

 from cells budded off from the mother mesoderm cells. 



Three sets of powerfully developed muscles are formed, consist- 

 ing of spindle-shaped cells which arise from the proliferation of the 

 mesoderm cells. All three are inserted into the cuticle of the hinge 



