2l8 MANUAL OF ZOOLOGY. 



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, five, or six pairs), and are mostly 

 respiratory, carrying the branchiae. Two pairs of antennse are 

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

 acting as natatory organs. The Gladocera quit the egg with the 

 full number of limbs proper to the adult. 



In the Daphnia pulex (fig. 66, b), or " branched-homed 

 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 protrusioii 

 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 maxilla. 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 fqjrth 

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

 ence of a single fecundation appears to be transmitted through 

 several generations. Two kinds of eggs occur in Daphnia. 

 In the first of these, or " summer eggs," the ova (from ten to 

 fifty in number) are deposited in an open space between the 

 valves, and are retained there till the young are ready to be 

 hatched. In the second of these, or "winter eggs," the ova 

 (generally two in number) are placed in a peculiar receptacle, 

 which is formed on the back of the carapace, and is called the 

 " ephippium " or saddle. After a time the ephippium is cast 

 offi and floats about till spring, when its contained eggs are 

 hatched by the warmer temperature of the water. 



Order II. Phyllopoda. — Crustacea, mostly of small size, 

 the carapace protecting the head and thorax, or the body 

 entirely naked. Feet numerous, never less thfn eight pairs, 

 mostly foliaceous or leaf-like, branchial in function. The eyes 

 sometimes confluent, sometimes distinct and sub-pedunculate. 

 There are two horny mandibles without palps, and the first pair 

 of feet are oar-like, with setiform terminal appendages. The 

 remaining feet are branchial, and adapted for swimming. The 

 Phyllopods undergo a metamorphosis, the youngest forms being 

 "Nauplii." In Nebalia, however, which is the only marine 

 Phyllopod, " Zoea-stages " are superadded as well. 



The Phyllopoda are chiefly interesting from their affinity to 



