ix MOLLUSC A 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 



-per 



. o nay 



int 



-Sr^*- <^r^> Z^K^ 



our 



int 



FIG. 282. Diagrams illustrating the development of the pericardium in t'yvlus and 

 1 ) reissensia. (After Meisenheimer. ) 



A, development of pericardium in Cydiis. B, development of pericardium in Drciw.nsia. rmr, space 

 which forms the auricle of the heart; //, heart; int, intestine ; i:r, pericardium. 



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

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

 development of the embryo of Unio conforms to the scheme given for 

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

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

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

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

 The group of small cells along its lower edge (Xj-x 5 ) 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 

 inesoderm 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 

 rneseuchyme. The shell gland is enormous and the endodermic 

 rudiment very small. As the shell gland becomes everted the 



