ANNULOSA: CRUSTACEA. 295 



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 first two groups. 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, five, or six pairs'), and are mostly respir- 

 atory, carrying the branchm. Two pairs of antenna are present, 

 the larger pair being of large size, branched, and acting as nata- 

 tory organs. The Cladocera quit the egg with the full number 

 of limbs proper to the adult. 



In the Daphnia pulex (fig. 145, 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 man- 

 dibles 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 pro- 

 duced from the original fecundated female are able to bring 

 forth young without having access to a male. 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 until 

 the young are ready to be hatched. In the second of these, 

 or "winter eggs," which alone are fecundated, the ova (gener- 

 ally two in number) are placed in a peculiar receptacle, which 

 is formed on the back of the carapace, and is called the " ephip- 

 pium" or saddle. After a time the ephippium is cast off, 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 than eight pairs, mostly foli- 



