M 



PHYLUM ARTHROPODA 



559 



(Fig. 444, TA\ and three pairs of elevations appear between it 

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

 paire of appendages, the antennules (a r ), antenna? (a. 2 .\ and man- 

 dibles (m.): 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 : it is the stomodccum (Fig. 455, 

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

 more cylindrical pit appears on the thoracico-abdominal rudiment : 

 it is the prododccum (pcdm.), and its aperture the anus. For a 



A, (above) eye ; A, (below) anus ; a^. anenuue 

 , mandibular ganglion ; 



antenuule ; 



FIG. 444. Nauplius-stage of Astacns. 



._>. antenna ; G, cerebral ganglion ; {/<>. antennary ganglion ; r/m, 



I." labrum ; m. mandible ; TA, thoracico-abdominal rudiment. (From Lang's Comparative 



Anatomy, after Reicheubach.) 



considerable time both stomodaeum and proctoda3um remain in 

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

 archenteron, a complete enteric canal being thus constituted. In 

 the meantime the endoderm cells lining the archenteron grow 

 outwards in a radial direction, ingesting the yolk as they do so, 

 until they take the form of long columns, in contact by their outer 

 ends with the ectoderm (Fig. 446, B). 



The thoracico-abdominal rudiment soon begins to increase 

 rapidly in length, but, being enclosed in the egg-membranes, it 

 grows not backwards but forwards, being in fact folded upon the 

 anterior part of the body in much the same way as the abdomen 

 of the adult during extreme flexion. Thus in Fig. 447 the ventral 

 surface of the head and anterior thoracic region faces the observer, 



