OUR FRESHWATER EUTOMOSTRACA. 
43 
These minute shell-insects, then, or Entomostraca, belong to 
the great class Crustacea , and, though possessing extreme deli- 
cacy of structure, are formed on the same model as our shrimps, 
prawns, and lobsters. They have a brain, or several nervous 
knots, called ganglia , which supply its place ; a heart, which is 
generally placed near the centre of the body, and propels the 
blood or circulating fluid to the different organs ; and gills, or 
branchiae , attached either to the feet or organs of mastication, by 
means of which respiration is carried on. They are all aquatic, 
and in general are covered externally by a shell, or what is 
called a carajpace , formed of one or two pieces, of a flexible 
horny texture. They have more or less well-formed organs of 
mastication, and have jointed ciliated feet, well adapted for the 
purpose of swimming. These creatures vary much, however, in 
their general appearance, and have been divided by naturalists 
into several large groups, or orders. We append a short de- 
scription of these in a note below. * * * § 
* Our freshwater shell-insects, living free and unattached in the water, 
are all contained in two great divisions, or legions, called — 
1. Braxchiopoda,* Branchiopods ; which are characterised by their 
having many gills or branchice, which are attached to the feet , these latter 
varying in number, in some being very numerous, and only formed for swim- 
ming ; by their body being either naked or having an envelope in form of a 
buckler, in some enclosing only the head or thorax, in others the whole 
body ; and by their mouth being furnished with organs fitted for mastication. 
II. LoPHYEOPODA,f Lophyropods ; which are characterised by their having 
few gills or branchice , which are attached to the organs of the mouth ; by having 
few pairs of feet, and these serving chiefly for purposes of locomotion ; by 
their body having always an envelope, either in form of a buckler enclosing 
the head and thorax or in shape of a bivalve shell enclosing the whole 
animal ; and by their mouth being furnished with organs of mastication. 
The first legion, the Branchiopods, are divided again into two orders : — 
. 1. Phyllopoda,! Phyllopods ; which are characterised by their having 
the body either naked or having only the head and thorax covered by the 
carapace ; by having numerous pairs of feet, the joints or articulations of 
which are foliaceoits or leaf-like, and perform the part of organs of respira- 
tion, instead of locomotion ; and by their feelers or antennae being small and 
not adapted for assisting the animal in swimming. 
This order contains the Apus and Chirocephalus. 
2. Cladoceea,§ Branched-horned Water-Fleas, which are characterised 
by their having the whole body, with the exception of the head, which is 
distinct and projecting, entirely enclosed within a carapace formed of two 
* Branchia (G.), gills j pous (G.), a foot, 
t Lophurus (G.), having stiff hairs ; pous (G.), a foot. 
| Phyllon (G.), a leaf ; pous (G.), foot. 
§ Klados (G.), a branch ; Keros (G.), a horn. 
