368 



CRUSTACEA EXTOMOSTRACA. 



mented, without free reduplicature of the skin (Branchipus, 

 Fig. 247), or it may be covered by a broad and flattened shield, 

 an extension of the cephalic integument, which only allows the 

 posterior part of the body to project uncovered (Apus, Fig. 248). 

 In other cases the body is laterally compressed and enclosed 



by a bivalve shell from 

 which the anterior part of 

 the head projects (Cladocera) 

 (Fig. 251) ; or finally the 

 head together with the 

 laterally compressed body 

 is completely covered by 

 a bivalve shell which is 

 closed by a shell muscle 

 passing ^between the valves 

 (Conchostraca). Sometimes 

 the head is sharply distinct 

 from the rest of the body, 

 and in one family, the Bran- 

 chipodidae, the anterior 

 part, bearing the eyes and 

 antennae, is divided by a 

 transverse groove from the 

 posterior, bearing the other 

 three pairs of cephalic ap- 

 pendages (Sars). As a rule 

 the posterior segments only 

 are without appendages. 

 The hind end of the body 

 is often curved forwards 

 and may bear two rows of 

 posteriorly directed claws, 

 the last pair of which arise 

 at the point of the tail, 

 and are by far the strongest 



(Cladocera). In other cases a pair of simple (BrancTiipus] 

 or articulated (Apus} appendages are present, constituting 

 the caudal fork. 



Appendages. On the head there are two pairs of antennae, 

 which however, in the adult animal, may be rudimentary or 



FIG. 247. Male of Branchipus stagnalis. Ry 

 heart or dorsal vessel with a pair of slit-like 

 ostia m each segment ; D intestine ; M man- 

 dible ; Sd shell gland ; Br distal branchial 

 appendages of the eleven pairs of legs ; Ttestis. 

 The anterior antenna is seen as a slender 

 appendage curving forwards in front of the 

 eye over the base of the strong, prehensile 

 posterior antenna. The long tentacular and 

 short foliaceous appendages of the latter are 

 also seen. 



