CRAYFISH. 295 
into themselves in 4 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. 
Fic. 148.—Longitudinal section of later embryo of 
Astacus.—After Reichenbach. 
£c., Ectoderm ; #z., mesoderm cells; ¢.g., cerebral ganglia; s¢., 
stomodeum; A., anus; 7., telson; g., ventral ganglia; s.s., 
sternal sinus ; Ad., proctodeum; %., heart ; #zg., mid-gut ; yolk 
pyramids dark. ‘ 
As usual, the nervous system arises from an ectodermic thickening. 
The eyé 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 
