154 CRUSTACEA. 



dorsal organ (d). The plate-shaped rudiment of the caudal region 

 («) is bounded anteriorly by the transverse depression (abdominal 

 fold). The latter soon becomes covered over by the posterior end of 

 the body (Fig. 77 B, s), which grows forwards to form the caudal 

 fork. In this way the Nauplius stage, with the tail region bent over 

 ventrally, is reached (Fig. 77 B). 



After this stage has been reached, the vitelline membrane is 

 thrown off, a new cuticle (the Nauplius integument) being secreted 

 simultaneously. The embryo now lies free in the brood-cavity, 

 surrounded somewhat loosely by the Nauplius cuticle. The abdomen 

 has meanwhile lengthened and straightened out (Fig. 77 6'), and 

 becomes more and more filled with food-yolk evenly distributed 

 within it, while the whole body finally assumes a distinct dorsal 

 curvature. We can now (as in the Arthrostraca) distinguish a ventral 

 germ -band from the dorsally-placed mass of food-yolk. The latter, 

 especially in the cephalic region, swells considerably. The posterior 

 parts of the embryo become distinctly segmented. The mouth - 

 parts and the thoracic limbs appear simultaneously (Fig. 77 E), 

 the rudiments of the abdominal limbs following in a later stage. 

 During these changes the disc-shaped dorsal organs have coalesced in 

 the dorsal middle line, and, as an invagination takes place there, they 

 are here, both in position and form, comparable with the dorsal organ 

 of the Amphipoda. 



The embryo of Nebalia, according to Metschnikoff's account (No. 33), 

 strongly resembles in form that of Mysis. Here also we have the ventrally 

 flexed Nauplius stage, as well as a lengthening and dorsal curvature taking 

 place after the shedding of the vitelline membrane. The presence of a dorsal 

 organ has, however, apparently not yet been observed in this form (p. 252). 



The embryos of the Decapoda differ from those of Mysis chiefly in 

 the position of the parts of the body ; the ventral curvature which at 

 first characterises the Nauplius of Mysis (i.e., the ventral flexure of 

 the abdomen) is here retained to a much later stage until the larva is 

 hatched, this being connected with the much later rupture of the 

 egg-envelopes. In other respects the ontogenetic processes are very 

 similar. In Astacus, of whose development a very detailed account 

 can be compiled from the researches of Rathke (No. 63), 

 Lereboullet (No. 58), Bobretzky (No. 41), and Reichenbach 

 (Nos. 64 and 65), after the blastoderm has developed, the first 

 embryonic rudiments are perceptible as five thickenings consisting 

 of a simple layer of cells (Fig. 78). The anterior pair of the 

 thickenings (K) corresponds to the optic rudiments of Mysis, and 



