XI 



PHYLUM ARTHROPOD A 



517 



Twmplius-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 antennae a pit appears, 

 which soon deepens and widens: it is the stomodceum (Fig. 411, 

 stdm.\ and its aperture the mouth. A similar but narrower and 

 more cylindrical pit appears on the thoracico-abdominal rudiment : 

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

 considerable time both stomodseum and proctodaBum 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 



I'K.. 410. Nauplius-stage of Astacus. A, (above) eye; A, (below) anus; ttj. anteuuule ; 

 i2- antenna ; G, cerebral ganglion ; git*, antenuary ganglion ; r/>n, mandibular ganglion ; 

 ?."labrum; HI. mandible; TA, thoracico-abdominal rudiment. (From Lang's Compamtire 

 Anatomy, after Reicheiibach.) 



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. 411, 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. 412 the ventral 

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

 but the dorsal surface of the posterior thoracic and abdominal 

 regions : in order to bring the parts into their adult position, the 

 abdomen must be supposed to be lifted up and turned backwards. 



