ISOPLEURA. 



391 



Generative organs. The sexes are separate, and the generative 

 gland is single and median, and occupies the dorsal part of the body 

 just in front of the pericardium, with which, however, it has no 

 connection. The generative ducts are paired, and arise from the 

 hinder end of its ventral surface; they are curved on themselves 

 near their origin, and in the female have a glandular enlargement on 

 their course. They open into the mantle-groove a little in front of 

 the renal openings (Fig. 308). 



fl. 



FIG. 309.^. larva of Chiton marginal us (after Loven). B embryo, and C larva of Chiton polii 

 (after Kowalevsky) from Korschelt and Heider. a eye ; fc rudiment of shell-plates ; in 

 mouth ; st spines ; w velum ; ws apical tuft of cilia. 



The eggs when laid have a spiny shell, and may be kept in 

 the mantle-groove or expelled freely into the sea. Fertilization is 

 external. 



In the development the young are hatched as larvae and pass 

 through a troch.osph.ere stage. There is no veliger stage or nautiloid 

 shell. The blastopore becomes the mouth. 



The Chitons are marine animals which live like limpets attached 

 to stones by the foot. They inhabit the littoral zone, and have 



