6o LILLIE. [Vol. X. 



mesodermic cells (myocytes ; products of the larval mesoblast), 

 some of which show a paired arrangement (Strangzellen of 

 authors). Stretching across the primary body cavity from one 

 shell valve to the other, nearer the anterior than the posterior 

 end, is the larval adductor muscle, likewise a product of the 

 larval mesoblast ; dorsal to this, and running parallel to the 

 hinge-line from the posterior end of the primary body cavity 

 to open anteriorly between the five large cells already noticed, 

 is the large unicellular thread-gland. 



The accompanying text figures (3, 4, and 5) will make the rela- 

 tions of the areas more readily referable to the cleavage stages. 

 It of course goes without saying that the areas in the second 

 and third figures are only approximately correct in outline. 

 The first figure is an actual reproduction of Fig. 47 (PI. IV). 



(b) Transformation of the Young Larva into the Glochidium. 



Perhaps the simplest way of describing the transformation 

 into the glochidium will be, first, to describe the glochidium, 

 and then to ask how this form is derived from that of the 

 young larva already described. Figs. 92 and 93 illustrate 

 this description. 



The glochidium larva of Anodonta possesses two triangular 

 shell valves joined by their bases at the hinge-line. The valves 

 are quite thick, strong, and brittle, and pierced by numerous 

 fine pores. At the apex of each valve is a strong hook (pro- 

 vided with numerous teeth), which is quite different in appear- 

 ance in Unio and Anodonta, being much stronger in the latter 

 form, which I have figured. These hooks are joined to the 

 valve proper by a hinge, and are moved by special muscles 

 (myocytes). Each valve is somewhat spoon-shaped, and the 

 cavity is lined by the larval mantle, consisting of large, flat 

 vacuolated cells. The curve of the anterior edges of the valves 

 is considerably greater than that of the posterior edges ; the 

 hinge-line straight, and of considerable extent. The valves are 

 united by a strong internal ligament. The adductor muscle is 

 very powerful, and, as in the young larva, is nearer the anterior 

 than the posterior end. The larval mantle bears four paired 



