ANNULOSA: CRUSTACEA. 183 



genera resembling a cuirass or buckler, and in others a bivalve 

 shell, which completely or in great part envelops the body 

 and limbs of the animal ; in other genera the animal is invested 

 with a multivalve carapace, like jointed plate- ar mour ; the 

 branchiae are attached either to the feet or to the organs of 

 mastication ; the limbs are jointed, and more or less setiferous. 

 The animals, for the most part, undergo a regular moulting 

 or change of shell, as they grow ; in some cases this amounts 

 to a species of transformation.' 



The Entomostraca are divided into two great divisions, or 

 ' legions,' the Lophyropoda and the Branchiopoda, with which 

 the order Merostomata may be conveniently considered. 



DIVISION A. LOPHYROPODA. The members of this division 

 possess few branchiae, and these are attached to the appen- 

 dages of the mouth. The feet are few in number, and mainly 

 subserve locomotion ; the carapace is in the form either of a 

 shield protecting the cephalothorax, or of a bivalve shell 

 enclosing the entire body. The mouth is not suctorial, but is 

 furnished with organs of mastication. 



This division comprises the two orders Ostracoda and 

 Gopepoda. 



ORDER I. OSTRACODA. Small Crustaceans having the entire 

 body enclosed in a shell or carapace, which is composed of two 

 valves united along the back by a membrane. The branchiae 

 are attached to the posterior jaws, and there are only two or 

 three pairs of feet, which subserve locomotion, but are not 

 adapted for swimming. 



The order includes the Cy prides, which are of almost uni- 

 versal occurrence in fresh water. The common Cypris is 

 completely protected from its enemies by a bivalve carapace, 

 which it can open and shut at will, and out of which it can 

 protrude its feet. 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. 



In the Cyclops, 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 



