206 MANUAL OF ZOOLOGY. 



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 Copepodd) or Oar-footed Crustaceans, are all of small 

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

 of Europe. By Professor Huxley the Ichthyophthira are 

 regarded 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 

 to 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 

 small Crustaceans, which have a distinct head, and have the 

 whole of the remainder of the body enclosed within a bivalve 

 carapace, similar to that of the Ostracoda. The feet are few 

 in number (usually four or five pairs), and are mostly respira- 

 tory, carrying the branchiae. Two pairs of antennae are 

 present, the larger pair being of large size, branched, and 

 acting as natatory organs. 



In the Daphnia pulex (fig. 63, b\ or "branched -horned 

 Water-flea," which occurs commonly in our ponds, the body 

 is enclosed in a bivalve shell, which is not furnished with a 

 hinge posteriorly, and which opens anteriorly for the protrusion 

 of the feet. The head is distinct, not enclosed in the cara- 

 pace, and carrying a single eye. The mouth is situated on the 

 under surface of the head, and is provided with two mandibles 

 and a pair of maxillae. The gills are in the form of plates, 

 attached to the five pairs of thoracic legs. The males are 

 very few in number, compared with the females, and a single 



