iJo THE CRAYFISIi chai?. 



Before long the blastopore closes, and a stottiodseum and 

 proctodagum (p. 204) are formed as invaginations of the 

 'ectoderm, which eventually communicate with the enteron, 

 ■forming a complete enteric canal (p. 357). On each side of 

 the mouth or aperture of the stomodaeal depression (Fig. 92) 

 three elevations appear, the rudiments of the anten- 

 nules (a^), antennse (a^), and mandibles (/«) : 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. 



Many allied forms are hatched in the form of a free-swimming larva 

 (compare pp. il and 340), to which the name nMiplhis is applied, 

 characterised by the presence of three pairs of appendages used for 

 swimming, and_becoming the antennules, antennse, and mandibles of 

 the adult. In the crayfish there is no free larva, and the nauplius 

 stage is passed through before hatching. 



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 

 thorax and abdomen ; and by the gradual differentiation of 

 the appendages. When hatched the young animal agrees in 

 all essential respects with the adult, but its proportions are 

 very different, the cephalothorax being nearly globular and the 

 abdomen small. For some time after hatching the young 

 crayfishes cling in great numbers to the pleopods of the 

 mother by means of the peculiarly hooked chelae of the first 

 pair of legs. 



All the members of the phylum Arthropoda (p. 346) are 

 characterised by each typical segment of the body bearing a 



