ANNULOSA: CRUSTACEA. 217 



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 CyclopidcB), 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), 

 and two terminal setag. Next the maxilte 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. 66, 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 

 which the ova remain till they are hatched. A single congress 

 with the male is apparently sufficient to fertilise the female 

 for life. 



The Copepoda, or Oar- footed Crustaceans, are all of small 

 size, and are of common occurrence in fresh water in all parts 

 of Europe. By good authorities the Ichthyophthira are re- 

 garded as merely Copepoda peculiarly modified to suit a life of 

 parasitism. 



Division B. Branchiopoda. — The Crustaceans included in 

 this division have many branchiae, and these are attached 

 ta the legs, which are often numerous, and are formed for 

 swimming. In other cases the legs themselves are flattened 

 out so as to form branchiae. The body is either naked, or is 

 protected by a carapace, which may enclose either the entire 

 body, or the head and thorax only. The mouth is provided 

 with organs of mastication. 



The Branchiopoda comprise the Cladocera, the Phyllopoda, 

 and probably the Trilobita, though this order departs in many 

 respects from the above definition. The Merostomata may 

 be considered along with these, though these, too, are in many 

 respects peculiar. 



Order I. Cladocera. —The members of this order are 



