CRUSTACEA 



351 



some related genera there is a remarkable system of closed blood 

 vessels without a heart. 



The blood IS a pale fluid, which bears leucocytes except in ostracods 

 and most copepods. It contains in the Malacostraca the copper- 

 containing respiratory pigment haemocyanin (p. 133). In various 

 entomostraca, notably in Lernanthropus , just mentioned, haemo- 

 globin has been found. 



r.ov. 



Fig. 234. A, Male reproductive organs of Astacus fluviatilis. From Howes. 

 r.t. right anterior lobe of testis ; med.t. median posterior lobe of testis ; vas de. 

 vas deferens ; op. external opening of vas deferens ; leg, right fourth ambula- 

 tory leg on which the vas deferens opens. B, Female reproductive organs of 

 Astacus fluviatilis. From Howes, r.od. right oviduct :'the left oviduct is shown 

 partly opened ; r.ov. right lobe of ovary ; l.ov. left lobe of ovary with the upper 

 half removed to show the ovarian cavity, which is the remains of the coelom 

 and into which the ripe ova drop; op. external opening of oviduct; leg, right 

 second ambulatory leg on which the oviduct opens. 



As is usual with animals that are free and active, the sexes are 

 separate in the great majority of the Crustacea, though the Cirripedia, 

 which are sessile, certain of the parasitic Isopoda, and a few excep- 

 tional species in other groups, are hermaphrodite. Parthenogenesis 

 takes place in many of the BranchioJDoda and Ostracoda, and in these 

 it is often only at more or less fixed intervals that sexual reproduction 

 occurs. The male is usually smaller than the female and in some 

 parasites is minute and attached to her body. He has often clasping- 

 organs for holding his partner, and these may be formed from almost 



