242 THE ANATOMY OF IXVERTEBKATED ANIMALS. 



of principal swimming appendages, the future antennae and 

 the mandibular palps, the latter eventually entirely dis- 

 appearing. There is a pair of small antennules, a pair cf 

 strong legs in the .place of the suckers, and, behind them, the 

 rudiments of the prehensile legs and the first pair of bira- 

 mous appendages, the others being rudimentary. 



Notodelphys, which may be found very commonly in the 

 branchial sac of Ascidians, closely resembles an ordinary 

 Copepod, except that it becomes much distorted, and that 

 it carries its ova in a chamber formed by the dorsum of the 

 carapace. 



However strangely modified the adult form may be (and 

 it must be remembered that it is always the female which un- 

 dergoes the greatest amount of Change), the larvae of all 

 these epizoic parasites resemble those of the ordinary free 

 Copepoda in possessing only two (Achtheres, Tracheliastes) 

 or three pairs of appendages (which appertain to the anterior 

 region of the head) ; and they are endowed with considerable 

 powers of locomotion. 



The Beanchiopoda. — The genera Nebalia, Apus, Bran- 

 chipus, Limnetis, Daphnia, and their allies, are usually di- 

 vided into two orders, the Phyllopoda and the Cladocera ; 

 but these pass into one another so gradually, and have so 

 many structural peculiarities in common, that the subdivision 

 of the group of Branchiopoda appears to me to be a step of 

 doubtful propriety. Closely resembling the lower Podoph- 

 thalmia* such as 3fysis, in some respects, these Crustaceans 

 are invariably distinguished from them by the possession of 

 a greater or less number of somites than twenty ; Nebalia, 

 which most nearly approximates the higher Crustacea, hav- 

 ing twenty-two somites. Furthermore the thoracic and ab- 

 dominal appendages of the Branchiopoda are, in the majority 

 of cases, more or less foliaceous, resembling in many respects 

 the anterior maxillipede of an Astacus, and being constructed 

 on essentially the same plan. 



Apus glacialis (Fig. 63) presents an elongated vermiform 

 body, terminated by two long, multiarticulate, setose styles, 

 and covered anteriorly by a great shield-like carapace, deeply 

 excavated behind. The posterior three-fifths of the carapace 

 are free, and merely overlap the segments of the body ; the 

 anterior portion, on the contrary, is united with and forms 

 the tergal surface of the corresponding region of the head ; 

 the free portion of the carapace shelves away laterally from a 



