IX MOLLUSCA 351 



provided with a sharp, inturned tooth. This larva is capable of only 

 a few spasmodic flappings of the valves of the shell, which propel it 

 through the water for a short distance. It is ejected from the gills 

 by the parent when a freshwater fish happens to pass in the vicinity, 

 and a successful larva contrives to fix itself on the gills or fins of the 

 passing fish by grasping them by means of the valves of the shell. The 

 bite of the valves stimulates growth of the soft vascular gill, so that 

 the Glochidium is soon enclosed in a cyst in which it completes its 

 development, and from which it emerges only when it has attained 

 the adult condition (Fig. 283). 



The best account of the early development of Unionidae has been 

 given by Lillie (1895), and the most recent worker at the post-larval 



H 





CPfA r=VPi P^, 



^ox:^ 



"P^^ ;s. 





fiG. 282. —Diagrams illustrating the development of tbe pericardium in Cyclas and 

 Dreissensia. (After Meisenheimer. ) 



A, development of pericardium in Cyclas, B, development of pericardium in Dreissensia. aur, space 

 wliich forms the auricle of the heart ; //, heart ; int, intestine ; per, pericardium. 



development is Harms (1909). Lillie's account is interesting in 

 making it quite clear that, in spite of its aberrant appearance, th* 

 development of the embryo of Unio conforms to the scheme given I'ot 

 Dreissensia. There is, it is true, no prototroch, and the first quartette 

 of micromeres divide only once or twice and form the " head vesicle." 

 On the other hand, as in Dreissensia, the first somatoblast, 2d (X), is 

 enormous, and it divides in exactly the same way as in Dreissensia. 

 The group of small cells along its lower edge (x^-Xj) give rise to 

 what Lillie calls the ventral plate, a thickened region of the ectoderm 

 from which the foot is formed in post-larval life. There is a primary 

 mesoderm cell, 4d, which divides into right and left halves, from each 

 of which a packet of cells is formed, parts of which break up into 

 mesenchyme. The shell gland is enormous and the endodermic 

 rudiment very small. As the shell gland becomes everted the 



