196 CRUSTACEA. 



mutual action of the limbs and the development of endites connected with this 

 •is thus a new acquisition peculiar to the Crustacea. On the other hand, the 

 epipodial appendages may with some probability be deduced from the dorsal 

 cirri of the Annelida, although this view meets with a difficulty in the late 

 appearance of these structures in the ontogeny of the Crustacea. We must not, 

 however, forget that the respii-atory requirements only, as a rule, increase when 

 a certain size of body is attained, and that this appears to explain why small 

 Crustacean larvae (as well as adults of small size) have no branchial appendages. 



3. Phyllopoda. 



A. Branchiopoda. 



The larvae of the Branchiopoda, after hatching, bear a general 

 resemblance to the Nauplius, but the body is divided into an anterior 

 cephalic section carrying the Nauplius limbs and a posterior abdominal 

 section. Since the rudiments of a number of body-segments are to 

 be found in the latter, this larval stage of Branchipus must be 

 described as a Metanauplhis (Fig. 93). This stage is very little 

 modified in the various Branchiopodan genera. The rudiment of the 

 dorsal shell-fold is, as a rule, still wanting, only developing at a later 

 stage. The further metamorphosis is exceedingly simple, new 

 segments being successively yielded by the posterior end of the 

 body, and the limbs budding out in the same order. An exception 

 to this regular course of development is found in the limbs of the 

 maxillary region which, in keeping with their slight development in 

 the adult, appear late. While the segmentation of the body thus 

 gradually approaches that of the adult, the paired compound eyes 

 develop as Avell as the shell-fold, which latter must be counted as 

 belonging to the maxillary region, the internal organs develop, 

 and the three pairs of Nauplius limbs degenerate in size and 

 become modified. 



As an example of the development of this group we may take 

 that of Apus, which is so Avell known through the investigations of 

 Zaddach (No. 31), Brauer (No. 18), and especially of Glaus (Nos. 

 20 and 21). The MetanaupUus of Apus (Fig. 93 A) is more or less 

 oval in shape, narrowing posteriorly ; when it leaves the egg it still 

 retains the original dorsal curvature of the body (Brauer) but soon 

 straightens out. The only internal organs which can be recognised 

 are the Nauplius eye and the intestine which widens anteriorly and 

 opens out in a depression at the posterior end of the body. The 

 three pairs of limbs are of the typical Nauplius form. The first 

 antennae {1), which are inserted at the sides of the large, helmet- 

 shaped upper lip, are simple unjointed rods each carrying two large 



