3^4 THE CRAYFISH CHAP. 



the blastoderm. From this the ectoderm and endoderm are 

 derived, the latter enclosing a relatively small archenteron, 

 formed by an in-pushing or imagination 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 accumulate 

 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 stomodaeal depression (Fig. 99) 

 three elevations appear, the rudiments of the anten- 

 nules (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 procto- 

 daeal 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 rudiment of 



