10 A HISTOKY OF EECENT CRUSTACEA 



in fresh water, where they are common, but inconspicuous, 

 and to the ordinary observer little suggestive of the crus- 

 tacean type. 4. The Branchiura, represented by the carp- 

 lice, are so designated from having a branchial tail which 

 actively assists in the function of respiration. 



The Ostracoda, a title which might be interpreted as 

 the testaceous Crustacea, may be easily mistaken for minute 

 bivalve mollusca. Like the Branchiopoda they are divided 

 into four sub-orders, the Podocopa, Myodocopa, Cladocopa, 

 and Platycopa, in which names words meaning feet, mus- 

 cles, branch, and broad, are respectively compounded with 

 the Greek word signifying an oar. 



The Copepoda point at once to a connection with the 

 preceding order, inasmuch as there one of the sub-orders 

 derived its name from words signifying a foot and an oar, 

 while the Copepoda are indebted, to the very same compo- 

 nents, in the inverse order of an oar and a foot. The 

 actual structure of the animals to some extent justifies 

 this similarity of names, but in general appearance the 

 Copepoda, not being shut up in two-valved shells, are 

 widely different both from the Podocopa and the rest of 

 the Ostracoda. Three sub-orders are formed : (1) the 

 Gnathostoma, having the mouth well provided with jaws ; 

 (2) the Poecilostoma, in which the mouth varies ; (3) the 

 Siphonostoma, having the mouth produced into a siphon 

 or tube. 



The Gigantostraca are as rare as the Entomostraca are 

 common. They are divided into three orders, the Mero- 

 stomata, Xiphosiira, and Trilobita. Of these, the first and 

 third are entirely extinct, so that the knowledge of them 

 is derived only from fossil remains. 



The Merostomata have a name derived from two words, 

 meaning a thigh and a mouth, this singular combination 

 alluding to the no less singular fact that in these animals 

 the mouth is surrounded by a group of limbs which are 

 not only locomotive and prehensile, but also subservient 

 to mastication. This peculiarity belongs likewise to the 

 Xiphosura, or sword-tails, which are named from the long 

 and sharp piece at the end of the body, their characteristic 



