BRANCHIOPODA. 



369 



peculiarly modified. The anterior antennae are small, and bear 



the delicate olfactory hairs. The posterior antennae frequently 



have the form of large biramous 



swimming appendages, but they 



may be modified as prehensile 



organs, e.g. in the males Bran- 



chipus and its allies (Fig. 247). 



In other cases (Apus) they are &./'< 



rudimentary (Fig. 248) and may 



even be entirely absent. 



A pair of large mandibles is 

 always present beneath the well 

 developed upper lip ; they possess 

 a toothed, biting edge, and in the 

 fully developed condition are 

 destitute of palps.* The man- 

 dibles are followed by one or 

 two pairs of slightly developed 

 maxillae. A kind of underlip is 

 in many cases present, in the 

 form of a bilobed prominence be- 

 hind the mandibles. 



FIG. 248. Apusglacialis, ventral aspect of 

 female, abd.f abdominal limbs ; ant.l 

 first antenna ; ant.2 second antenna ; Ibr 

 labrum ; md mandible ; mx first maxilla ; 

 ov brood pouch ; s.f.pl sub-frontal plate ; 

 sh.ql shell-gland ; th.fl and th.f thoracic 

 feet. (From Parker and Haswell, after 

 Bernard.) 



It is remarkable that while the tho- 

 racic appendages of the Branchiopods 

 conform so uniformly to a common 

 and, as it appears, primitive type of 

 structure, the appendages about the 

 mouth present the greatest divergence 

 from that type which is met with in any group of Crustacea. The absence 

 of the mandibular palp in the adult is all the more striking because of its 

 size and frequently biramous character in other Entomostracan groups, 

 the Copepods and Ostracods. Again, the two pairs of maxillae, which 

 even in the Malacostraca conform more closely than any other appendage 

 to the primitive " Phyllopod " type of limb, are here, in the Branchiopods 

 themselves, perfectly simple lobes, and one pair is often missing. In 

 adult Cladocera and in Limnetis among the Conchostraca there is only 

 one pair, in Branchipus the second pair is rudimentary, and in this genus, 

 as also in Apus where two pairs are present, they are retarded in develop- 

 ment, appearing later than the thoracic appendages which follow them 

 in position. It is interesting to note that in Cypris, among -the Ostracods, 

 the second maxillae are also retarded in their development. The late 

 appearance of the maxillae is perhaps a confirmation of the view taken 



* Ekman finds a vestigial palp in Polyartemia forcipata (Bih. K. svenska 

 vet Akad. Handlingar, Bd. 28 [1902]). 



Z III B B 



