370 THE CRAYti"Isn CHArt 



Before long the blastopore closes, and a stomodaeum and 

 proctodeum (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 stomoda;al depression (Fig. 92) 

 three elevations appear, the rudiments of the anten- 

 nules (rt^), antenna; (ct^), and mandibles (///) : in front of 

 them is another pair of elevations on which the eyes (A) 

 subsequently appear. i\n unpaired elevation (TA) behind 

 the mouth, having the anus or aperture of the procto- 

 dajal depression at its summit (A), is the rudiment of 

 the thorax and abdomen. The embryo is now in the 

 iiauplius stage. 



Many allied fonns art hatched in the form of a free-swimming larva 

 (compare pp. 11 and 340), to which the name naupUiis is applied, 

 characterised l)y the presence of three pairs of appendages used for 

 swimming, andbccoming the antennules, antenna;, 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 ajjpearance 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, ^\'hen 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 chela: of the first 

 pair of legs. 



All the members of the phylum Arthropoda ([i. 346) are 

 characterised by each typical segment of the body bearing a 



