156 



CRUSTACEA. 



and the two pairs of antennae, the former, according to BOBRETZKY 

 and REICHENBACH, appearing somewhat earlier than the latter. The 

 middle region between these rudiments long retains the character of 

 an undifferentiated blastoderm ; in the anterior region, however, an 

 unpaired swelling arises as the rudiment of the Idbrum (/), and 

 behind it the stomodaeal invagination, while, mesial to the rudiments 

 of the limbs, the pairs of ganglia belonging to them (ga 2 , gm) are 

 recognisable as ectodermal thickenings. The Nauplius stage thus 

 attained marks a natural period in the ontogenetic development of 



Astacus, this being 

 also indicated by 

 the appearance of a 

 larval integument 

 (Nauplius cuticle). 



It should be noted 

 that, in the Nauplius 

 stage, the different 

 parts of the embryo 

 lie nearer one another 

 than when they first 

 arise (cf. Figs. 78 and 

 79 drawn to the same 

 scale). Such an ap- 

 proximation of the 

 rudiments in the em- 

 bryo at these stages 

 appears universal in 

 the Decapoda. 



The Nauplius stage 

 in other Decapoda 

 develops in the same 

 way as in Astacus. 

 The embryonic rudi- 

 ments, however, in 

 most cases, only ap- 

 pear after the closing 



of the mouth of the gastrula ; there is then found, in the immediate neigh- 

 bourhood of the closed blastopore, an unpaired rounded prominence, in which 

 the rudiment of the posterior end of the body can be discerned ; this is not, 

 as in Astacus, paired. In the Loricata, according to DOHRN (No. 45), as in 

 Astacus, this may be described as the thoraco-abdominal rudiment, whereas, in 

 other cases, it seems to give origin exclusively to the abdominal segments. This 

 rudiment soon shows wing-like outgrowths running forward, and establishing 

 connection with the optic lobes which in the meantime have made their appear- 

 ance. The rudiments of the Nauplius limbs then appear in the neighbourhood 

 of these connecting strands. 



A transverse projection connecting the posterior parts of the optic lobes 



FIG. 79. Embryo of Astacus fluviatilis in the Nauplius stage (after 

 REICHENBACH, from LANG'S Text-book). A (above), rudiment of 

 e ye > n first, and a 2 , second antenna ; G t cerebral ganglion ; 

 ga 2 , ganglion of the antenna ; gm, ganglion of the mandible ; 

 m, mandible ; t, upper lip ; TA, thoraco-abdominal rudiment ; 

 A (lying in TA), anus. 



