No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 



71 



efficient parasite ; and second, so as to be protected, as far as 

 possible, during its brief but necessary contact with the fresh 

 water. To ensure the continuance of the species an enormous 

 number of young would have to be produced. With the aid of 

 some such an hypothesis the curious ontogeny of the Unionidae 

 becomes more comprehensible. 



(c) Axial Relations, 



I have left till the last the consideration of the trochophore 

 stage, and the axial relationships of the larva in the Unionidae, 

 because it seemed better to have the whole course of develop- 



vp 



Fig. 6. — Gastrula of Uiiio. 



Fig. 7. — Slightly older Gastrula of Unto. 



Fig. 8. — Cyclas (after Stauffacher) 



Bp. Blastopore. 

 h.v. Head-vesicle, 

 o.p. Oral plate. 



v.p. Ventral plate. 

 Ent. Entoderm. 

 M. Mesoblast. 



Teredo (after Hatsthek). 



oes. Oesophagus, 

 s.g. Shell-gland. 

 Y. Larval Mesoblast. 



