242 ARTHROPODS. 



invagination occurs, representing the gastrula, whose typical 

 formation is prevented by the quantity of yolk present. 

 By this invagination a definite inner (endoderm) layer is 

 established, and this is very soon accompanied by the begin- 

 ning of a middle (mesoderm) stratum. 



The stage of development at which three distinct 

 appendages are developed is interesting, because it cor- 

 responds to the Nauplius, at which most of the lower 

 Crustaceans , and even the shrimp PencBus begin their free 

 life. At this stage, in Asfacus, a cuticle is formed and 

 moulted. Soon, however, the masticatory appendages, then 

 the walking limbs, and finally the abdominal parts are 

 formed. 



Derivatives of the epiblast or ectoderm. — (i) This layer 

 forms the epidermis, its cuticle, and the inward prolongations 

 of this. (2) The nervous system begins as an ectodermic 

 thickening. (3) The eye arises (a) from the optic ganglion, 

 developed in connection with the brain, {b) from an eye-fold 

 or invagination, as the result of which the retinulge are 

 formed, and (c) from the epidermis, which forms the 

 crystalline cells. The ear is formed from an invaginated 

 sac. (4) The gills are in great part ectodermic outgrowths. 

 (5) The green-gland is another invagination. (6) The 

 hind-gut and the fore-gut are invaginations from opposite 

 ends, meeting the endodermic gastrula cavity. 



Derivatives of the hypoblast or endoderm. — From the 

 cells invaginated into the yolk, at first as an open, but soon 

 as a closed sac, the mid-gut is developed. The future 

 digestive function is early suggested by the way in which 

 the endoderm cells devour the surrounding yolk elements. 

 From the primitive mid-gut the digestive gland is budded 

 out. 



Derivatives of the Mesoderm — The heart, its blood-vessels, 

 the blood, the muscles, are due to this layer. 



When the young crayfish are hatched and freed from 

 their surrounding husks, they still cling to these, and thereby 

 to the swimmerets of the mother. In most respects they 

 are miniature 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 attach- 

 ment, and there are other j^slight differences. The note- 



