CRA YFISH. 295 



into themselves in a way which early suggests their future digestive 

 function. On the surface of the egg there have already appeared 

 ectodermic thickenings, the so-called eye-folds, rudiments of the 

 appendages, and of the thoracic and abdominal regions. 



In the later stages invaginations of the ectoderm form the fore- and 

 hind-gut, which grow inward from opposite ends to meet the endo- 

 dermic mid-gut. The ear-sac and the greater part of the gills have 

 also an ectodermic origin. From the mid-gut the digestive gland is 

 budded out. The heart, the blood vessels, blood, and muscles are due 

 to the mesoderm. 



FIG. 148. Longitudinal section of later embryo of 

 Astacus. After Reichenbach. 



EC., Ectoderm; *., mesoderm cells; e.g., cerebral ganglia; */., 

 stomodaeum ; A., anus; T., telson;^-., ventral ganglia; s.s., 

 sternal sinus \pd., proctodaeum ; k. % heart ; mg. t mid-gut ; yolk 

 pyramids dark. 



As usual, the nervous system arises from an ectodermic thickening. 

 The eye arises partly from the optic ganglia of the "brain," partly from 

 the "eye-folds," and partly from the epidermis. 



When the young crayfishes are hatched from the egg-shells, they still 

 cling to these, and thereby to the swimmerets of the mother. In most 

 respects they are like the adults, but the cephalothorax is convex and 

 relatively large, the rostrum is bent down between the eyes, the tips of 

 the claws are incurved and serve for firm attachment, and there are 

 other slight differences. The noteworthy fact is that the development 

 is completed within the egg-case, and that it is continuous without 

 metamorphosis. The shortened life history of the crayfish is interesting 



