ANNULOSA I CRUSTACEA. 235 



by means of an adductor muscle. Locomotion is mainly 

 effected by means of a pair of caudal appendages. The Cypris 

 is extremely prolific, and a single impregnation appears to last 

 the female for its entire lifetime. It appears, also, that the 

 young females, produced in this way, are capable for some 

 generations of producing fresh individuals without the influence 

 of a male (parthenogenesis). 



ORDER II. COPEPODA. Small Crustaceans, having the head 

 and thorax covered by a carapace, and furnished with five 

 pairs of natatory feet. Usually there are two caudal locomo- 

 tive appendages. A distinct heart is sometimes absent (as in 

 the Cjd&pidae), but is sometimes present. Both marine and 

 fresh-water Copepods are known. 



The larvae of the Copepods are Naupliiform, with unpaired 

 eyes, three pairs of limbs (the future antennae and mandibles), 



Fig. 87. Fresh-water Entomostraca. a Cypris tris-striata ; b Daphnia 

 pulex : c Cyclops qundricornis. 



and two terminal setae. Next the maxillae are produced, and 

 then three other pairs of limbs (the foot-jaws and the two front 

 pairs of natatory feet). At the next moult, the larva assumes 

 the Cyclops form, but has at first much fewer limbs and somites. 

 In the Cyclops (fig. 87, c\ which is one of the commonest of 

 the " Water-fleas," the cephalothorax is protected superiorly by 

 a carapace, and the abdominal somites are conspicuous. In 

 front of the head is situated a single large eye, behind which 

 are the" great antennae and the antennules. The feet are five 

 pairs in number, each consisting of a protopodite and a seg- 

 mented exopodite and endopodite, usually furnished with hairs 

 and forming an efficient swimming apparatus. The young 

 pass through a metamorphosis, and are not capable of repro- 

 ducing the species until after the third moult or change of 

 skin. The female Cyclops carries externally two ovisacs, in 



