10 A HISTORY OF RECENT CRUSTACEA 
in fresh water, where they are common, but inconspicuous, 
and to the ordinary observer little suggestive of the crus- 
tacean type. 4. The Branchitira, represented by the carp- 
lice, are so designated from having a branchial tail which 
actively assists in the function of respiration. 
The Ostracdda, a title which might be interpreted as 
the testaceous Crustacea, may be easily mistaken for minute 
bivalve mollusca. Like the Branchiopdéda they are divided 
into four sub-orders, the Podocdpa, Myodocépa, Cladocdpa, 
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 
Gnathost6ma, having the mouth well provided with jaws; 
(2) the Pcecilostéma, in which the mouth varies; (3) the 
Siphonostéma, having the mouth produced into a aa 
or tube. 
The Gigantostraca are as rare as the Entomostraca are 
common. ‘They are divided into three orders, the Mero- 
stomata, Xiphostra, 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 
