52 LAMELLIBEANCHIA UNIONIDAE. 



larva (Figs. 15 and 18). The broad part of the body in the embryo 

 of Anodonta which lies in front of the shell and of the entodermal 

 vesicle would correspond to the velum, i.e., to the pre-oral part of 

 the Trochophore. In the younger embryo depicted in Fig. 22, this 

 part appears to be formed solely by a somewhat thin layer of cells, 

 while, in the embryo represented in Fig. 24 A, it has a thick wall, 

 consisting of cells containing vacuoles such as ZIEGLEE has described 

 in the reduced velum of Cydas. The shape of this part of the 

 embryo recalls the swollen pre-oral portion or cephalic vesicle of the 

 Gastropodan embryos, a condition still more marked in them than in 

 Cydas, as ZIEGLEE pointed out. [LiLLiE also regards this area as 

 the head-vesicle]. 



In this region of the embryo, the polar bodies are occasionally met 

 with (Figs. 22 and 23 C) and these afford an indication for the 

 correct orientation of the embryo which is not otherwise very easy to 

 determine, and which was usually misinterpreted by the earlier 

 investigators. 



B. The Development of the Embryo into the Parasitic Larva. 



It is evident from the above that the peculiarities in the develop- 

 ment of the Unionidae appear very early and affect both the inner 

 and the outer organisation of the embryo. In the stage up to which 

 we have followed its development, it resembles a rounded vesicle of 

 somewhat irregular form consisting of a single layer of ectoderm on 

 the inner side of which there appear here and there single muscle-like 

 cells ; these belong to the mesoderm, the cells of which have increased 

 in number, some becoming lengthened (Figs. 23 and 24). In this 

 young embryo, the shell at first lies like a saddle upon the dorsal 

 side (Fig. 23 (7, and 24, ). 



The rudiments of the shell-valves appear later beneath the 

 unpaired cuticular shell. The shell probably arises here in the same 

 way as in Cydas ; each shell- valve in the Unionidae appears to be 

 three-sided and has its ventral point bent like a hook, a modification 

 connected with the manner of life of these forms. The shell carries 

 on its outer surface a number of small booklets which, together with 

 the two terminal hooks just mentioned, serve for attaching the 

 Lamellibranch to the body of its host during its parasitic life (Figs. 

 25 and 26, sh). Before the shell has developed thus far, a radical 

 transformation of the whole body takes place. The ventral part 

 of the body, which was not previously covered by the shell but 



