INVEETEBEATA 



CHAP. 



In the next stage the yolk has been completely ingested by 

 the endoderm cells, which have become in consequence very tall and 

 columnar in shape, and the fold which gives origin to the carapace 

 is strongly marked. Beneath it there is a deep groove on each side 

 which gives rise to the branchial cavity of the adult. Five 

 additional pairs of appendages, the rudiments of the so-called ambu- 

 latory legs, the pos- 

 session of which causes 

 Astacus to be reckoned 

 as a Decapod, are de- 

 veloped, and the first 

 and second maxilla and 

 the three maxillipedes 

 have become bifurcated. 



Fig. 137. — Advanced embryo of Astacus jiumatilis 

 viewed from the ventral side. The abdomen and 

 hinder part of the thorax are cut off and spread out 

 separately. (After Beichenbaoh.) 



ab, abdomen ; hr.c, rudiment of branchial cavity ; di.c, 

 deuterocerebrum ; ex, opening of excretory organ ; lab, labrum ; 

 ■pr.c, protocerebrum ; oc, eye-stallc ; th, thoracic legs; th.g, 

 thoracic ganglia ; tr.c, tritucerebrum. 



Fig. 138. — Two ommatidia from 

 the eye of a newly-hatched 

 crayfish in longitudinal sec- 

 tion. (After Reichenbach.) 



com, cornea ; eorn.c, corneal cells ; 

 crys, crystalline cone ; crys.c, crys- 

 talline cone cells ; rh, rhabdome 

 shimmering through the retinula ; 

 ^g, pigment cells. 



In the basal part of the rudiment of the antenna a sac appears, 

 which is the rudiment of the excretory organ {ex, Fig. 137). This 

 sac appears to be similar to one of the coelomic sacs of the abdomen, 

 to it is added a large ectodermic pocket which forms the thin-walled 

 ureter. 



Soon afterwards the rudiments of the abdominal feet, or pleopods, 

 make their appearance ; and in the bifurcated antennule there is an ecto- 

 dermic pit to be seen, which is the beginning of the auditory organ. 



