426 CRUSTACEA. 



knots. Hence the nervous system of these Crustacea is the most simple of all [yet 

 examined] . 



The branchiae 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 corauose the two other orders, Lcemodipoda and Isopoda. Cyumus, a parasitic 

 genus, belonging to the second of these orders, conducts us naturally to Bopyrus and 

 Cymothoa, with which we commence the aiTangement 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 ))re- 

 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 articidations, bifurcations, and the hairs or ciliae with which they are provided, 

 false legs or swimming fin-feet. In the Malacostraca belonging to tlie following orders, 

 these appendages have the form of plates or scales, and these hairs or ciliae ap])eai' to 

 constitute the brancliiffi. Many exhibit, as well as the Stomapoda and Lcemodipoda, 

 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 alwaj's 

 affixed to a distinct segment, being the one immediately behind the head. The antenna; 

 (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 

 ] \ \ v^ small articulated styles. The majority of these Crustacea swim 

 and leap with agihty, and always on then- sides, borne are lound 

 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. 



They 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, Westvv.] is divisible into two subsections : — 



