THE BRANCBIOPODA. 247 



portion of males to females yet known, viz., 33 in 72. On 

 the other hand, between 1857 and 1869, Von Siebold ex- 

 amined many thousands of specimens of the Bavarian Apus 

 without finding a single male. 1 



The testis is similar to the ovary in form, and its duct 

 opens upon the eleventh pair of*appendages, as in the case of 

 that of the female organs. The spermatozoa are oval and 

 without motion. 



The young Apus (cancriformis), when just hatched, is a 

 JVauplius. The body is oval, indistinctly divided into a few 

 segments, and entirely destitute of appendages, except a 

 shorter anterior, uniramous, and a longer posterior, biramous, 

 pair of oar-like organs, situated at the anterior extremity, on 

 either side of the single median eye. The carapace is rudi- 

 mentary, and there are no caudal filaments. The little ani- 

 mal soon casts its skin, and the mandibles, which are provided 

 with long palps, make their appearance. 2 . With successive 

 ecdyses, the larva assumes more and more the form of the 

 adult, and acquires the pair of compound eyes ; the anterior 

 pair of appendages being converted into the antennules, the 

 posterior pair disappearing, or remaining as rudimentary an- 

 tennae, and the mandibular palps also vanishing. 



Singular and highly instructive modifications are exhib- 

 ited by the other genera of the Dranrhiopoda, such as JYeba- 

 lia, Branchipus (C heirocephalus), Limnetis, and Daphnia. 



In Daphnia and its allies (Fig. 04), the thoracic members 

 are reduced to six, five, or even four pairs, some or all of 

 which may take the form of ordinary limbs ; the abdomen is 

 rudimentary ; the heart is short ; and the carapace presents 

 a posterior division (omostegite), obviously developed from 

 the anterior thoracic somites, the lateral halves of which are 

 deflexed so as to resemble a bivalve shell, into which the 

 hinder part of the body can be withdrawn. The anterior 

 division of the carapace (cephalostegite) in Daphnia has, on 

 the contrary, the same structure as the corresponding part of 

 the carapace of Apus, but the compound eyes, represented 

 by a single mass, are situated at the anterior extremity of 

 the head, rather than on its upper surface, and the single eye 

 is quite distinct, and far posterior to them (Fig. 64, B, I , 

 n"). The antennules (Fig. 64, A, n') are small, rudimentary, 



1 " Beitrage zur Parthenogenesis der Artbropoden."' 1871. It appears that, 

 in Apus, the impregnated ova alone give rise to males. 



2 According to Claus's recent investigations, this third pair of appendages 

 is present from the time the young Apus leaves the egg. 



