XI 



PHYLUM ARTHROPODA 



545 



(Fig. 451, c. I.), on which the eyes subsequently appear ; two 

 somewhat further back, the thoracico-abdominal rudiments (ih. abd) ; 

 and one, posterior and unpaired, the endoderm-disc (inv). On the 

 latter an invagination of the blastoderm takes place, giving rise to a 

 small sac, the archenteron, which communicates with the exterior 

 by an aperture, the blastopore. By this process the embryo passes 

 into the gastnda -stage, which, however, differs from the corre- 

 sponding stage in the types previously studied in the immense 

 quantity of food-yolk filling up the space (blastoccele) between 

 ectoderm and endoderm. Very soon the embryo becomes triplo- 

 blastic, or three-layered, by the budding off of cells from the 

 endoderm in the 

 neighbourhood of 

 the blastopore : 

 these accumulate 

 between the ecto- 

 derm and endo- 

 derm, and consti- 

 tute the meso- 

 derm. 



Before long the 

 blastopore closes, 

 converting the 

 archenteron into 

 a shut sac (Fig. 

 453, A) : the thora- 

 cic o - abdominal 

 rudiments unite 

 with one another, 

 forming a well- 

 marked oval ele- 

 vation (Fig. 452, 

 ih. abd), and three 

 pairs of elevations 

 appear between it 

 and the head-lobes. These are the rudiments of the first three 

 pairs of appendages, the antennules (at 1 .), antennae (at.' 1 ), and 

 mandibles (mn.) : by their appearance the embryo passes into the 

 nauplius- stage, which in this case is passed through in the egg, 

 instead of being active and free-swimming as in Apus. 



Between the bases of the antennules and antenna a pit appears, 

 which soon deepens and widens : this is the stomodceum (Fig. -1 ':'>, 

 stdm.), and its aperture the mouth. A similar but narrower and 

 more cylindrical pit appears on the thoracico-abdominal rudiment : 

 this is the proctodceum (pcdm.), and its aperture the anus. For a 

 considerable time both stomodseum and proctodaeum remain in 

 the condition of blind sacs, but after a time they open into the 



lhabd 



car 



m 



an 



FIG. 452. " A'auplius stage" in the development of Astacus 

 fluviatilis viewed from the ventral side. an. anus ; at 1 , 

 rudiment of antennule ; at-, rudiment of antenna ; car. ridge 

 marking the first trace of the carapace ; c.l. cephalic lobe ; lab. 

 labrum ; m. mouth ; mn. rudiment of mandible ; pr.c. proto- 

 cerebrum ; th.ab/i. thoracico-abdominal rudiment. (From 

 MacBride, after Eek-henbach.) 



