CRUSTACEA. 
426 
knots. Hence the nervous system of these Crustacea is the most simple of all [yet 
examined] . 
The branchise appear to be always attached to the two first appendages of the 
under- side of the abdomen. The female carries her eggs beneath the breast, between 
certain scales, which form a kind of pouch. They are there hatched, and the young 
ones remain attached to the legs, or other parts of the body of their parents, until they 
gain sufficient strength to swim and take care of themselves. These Crustacea are of 
small size, and reside for the most part either upon the shores of the ocean or in fresh 
water. Some are terrestrial and others are parasites. 
These animals are divisible into three orders : those in which the mandibles are fur- 
nished with a palpus, appear to be more nearly allied in nature to the preceding 
Crustacea — these are the Amphipoda. Those in which these organs are destitute of 
palpi compose the two other orders, Lcemodipoda and Isopoda. Cyamus, a parasitic 
genus, belonging to the second of these orders, conducts us naturally to Bopyrus and 
Cymothoa, with which we commence the arrangement of the Isopoda. 
THE THIRD ORDER OF CRUSTACEA, 
[the first of the Malacostraca Edriopthalma] or the AMPHIPODA,— 
Are the only Malacostraca with sessile and fixed eyes, of which the mandibles, as in the pre- 
ceding Crustacea, are furnished with a palpus, and they are the only order in which the 
subabdominal appendages, always very apparent, resemble, in their long and narrowed 
form, their articulations, bifurcations, and the hairs or cilise with which they are provided, 
false legs or swimming fin-feet. In the Malacostraca belonging to the following orders, 
these appendages have the form of plates or scales, and these hairs or cilise appear to 
constitute the branchiae. Many exhibit, as well as the Stomapoda and Loemodipoda, 
vesicular bags, placed either between their feet or at their base externally, and of which we 
are ignorant of the uses. 
The first pair of legs, or that which corresponds with the second pair of foot-jaws, is always 
affixed to a distinct segment, being the one immediately behind the head. The antennae 
(with the exception of the single genus Phronima) are four in number. They are advanced 
in front and gradually attenuated, terminating in a point, and composed, as in the preceding 
Crustacea, of a peduncle and a single terminal filament, (or accompanied sometimes by a 
small lateral branch) and generally multiarticulate. The body 
is ordinarily compressed, and bent downwards behind. The 
appendages at the extremity of the tail most frequently resemble 
small articulated styles. The majority of these Crustacea swim 
and leap with agility, and always on their sides. Some are found 
in brooks and fountains, often united in pairs, but the greater number inhabit the salt water. 
They are of an uniform colour, varying from reddish to green. 
Thev may be comprised in the single genus Gammarus, Fab., which may be distributed into 
three sections, from the form and number of the legs : — 
1. Those which have fourteen feet, all of which are terminated by a hook or a point. 
2. Those which have also fourteen feet, but in which these organs, or at least the four 
posterior, are unarmed and merely natatorial. 
3. Those which have only ten feet. 
The first of these sections [Homopoda, Westw\] is divisible into two subsections : — 
Fig. 11. — Gammarus pulex. 
