236 MANUAL OF ZOOLOGY. 



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 

 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 

 smaH 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 Ostrocoda. The feet are few 

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

 respiratory, carrying the branchiae. Two pairs of antennae are 

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

 acting as natatory organs. The Cladocera quit the egg with 

 the full number of limbs proper to the adult. 



In the Daphnia pulex (fig 87, 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 

 congress is all that is required to fertilise the female for life. 

 Not only is this the case, but the young females produced from 

 the original fecundated female appear to be able to bring forth 

 young without having access to a male. In this way the influ- 

 ence of a single fecundation appears to be transmitted through 



