346 



INVEETEBEATA 



CHAP. 



kidney which was originally the outer limb, bends inwards and fuses 

 with the lower part of the pericardium, andliere the reno-pericardial 

 canal is formed (Fig. 277).; 



Fig. 276. — Horizontal sections tlirougli the anterior portions of three just metamorphosed 

 specimens of Dreissensia polymorpha in order to show the transformation of the 

 cerebral pit into the labial palps. (After Meisenheimer. ) 



A, specimen in which the velum is just being thrown oif. B, old specimen in which the velum is 

 already discarded, in which the cerebral pit is beginning to open out. C, specimen in which the cerebral 

 pit has given rise to two ciliated lobes, eg, cerebral ganglion ; cj), cerebral pit ; l.%t, labial palps ; m.c, 

 mantle-cavity ; oes, oesophagus ; V, discarded velar cells. , 



The genital organ arises from a median ventral strip of the 

 pericardial wall, in front, just between the openings of the reno- 

 pericardial canals. It consists of peculiar large cells with pale nuclei. 

 These cells multiply, become detached from the pericardial wall, and 



J J 



