384 THE CRAYFISH CHAP. 



blastoderm. From this the ectoderm and endoderm are 

 derived, the latter enclosing a relatively small enteron 

 formed by an inpushing or invagination of the blastoderm 

 into the yolk, and communicating with the exterior 

 through a blastopore (p. 201). 



Very soon the embryo becomes triploblastic, or three- 

 layered, by the budding off of cells from the endoderm 

 in the neighbourhood of the blastopore : these accumu- 

 late between the ectoderm and endoderm, and constitute 

 the mesoderm. 



Before long the blastopore closes, and a stomodaeum 

 and proctodaeum (p. 204) are formed as invaginations of 

 the ectoderm, which eventually communicate with the 

 enteron, forming a complete enteric canal (p. 371). On 

 each side of the mouth or aperture of the stomodgeal 

 depression (Fig. 99) three elevations appear, the rudi- 

 ments of the antennules (a 1 ), antennae (a 2 ), and mandibles 

 (m) : in front of them is another pair of elevations on 

 which the eyes (A) subsequently appear. An unpaired 

 elevation (TA) behind the mouth, having the anus or 

 aperture of the proctodaeal depression at its summit (A), 

 is the rudiment of the thorax and abdomen. The 

 embryo is now in the nauplius stage. 



In many allied forms the embryo is hatched at this stage 

 in the form of a free-swimming nauplius larva (Fig. 100), 

 characterised by the presence of three pairs of appendages 

 used for swimming (the first simple, the second and third 

 forked), and becoming the antennules, antennae, and 

 mandibles of the adult : the eye is median and sessile. 

 In the crayfish there is no free larva, and the young are 

 hatched at a much later stage. 



The embryo is gradually transformed into the crayfish 

 by the appearance of fresh appendages, in regular order, 

 behind the first three ; by the elongation of the rudi- 

 ment of thorax and abdomen ; and by the gradu- 



