426 CRUSTACEA. 



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

 examined] . 



The hranchia: appear to he always attached to the two first appendages of the 

 under-j-ide of the abdomen. The female carries her egrgs Ix-neath 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 s\itiicient strength to swim and take care of themselves. T'hese 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 jial];us, appear to be more nearly allied in nature to the preceding 

 Crustacea — these are the Ajirniroii.\. Those in which these organs are destitute of 

 palpi comuose the two other orders, LcEMoniPou.i and Isopoda. Cyiiiiinf:, a parasitic 

 genus, belonging to the second of these orders, conducts us naturally to Bojti/rus and 

 Cymothuc , with which we commence the arrangement of the Isojioda. 



THE THIRD ORDER OF CRUSTACEA, 



[flip first of tlic Malacostraca I'.DRiorTHALMA] 01' tiip AMPHirODA, — 



Are the only Jlaliicostriica ^vith sessile and fixed eyes, of which the mandibles, as in the ])re- 

 cednig Crustacea, are furnished with a pul|)us, and they are the only order in \vhich the 

 suhabdduiinal apjiendages, always very apparent, resemble, in their long and narrowed 

 form, their articulations, bifurcations, and the hairs or cilia; 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 hiiirs or cilia; appear to 

 constitute the I)rancliia\ Many exliibit, as well as the Stomapoda and Lcemodipoda, 

 vesicular bags, placed either between their feet or at their base externally, aiul of \ihieh we 

 are ignorant of the uses. 



The first pair of Irgs, or that whieli corrcsjwnds with the second pair of foot-jaws, is always 

 affixed to a distinct segment, being the one immediately behind the bead. 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 ])oint, 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 multiartienlate. The body 

 is ordinarily compressed, and bent downwards behind. The 

 apjiendages at the extremity of the tail most frequently resemble 

 small articulated styles. The majority of these Crustacea swim 

 .g. . ,u.„niiTu.i,ii I, . and leap wath agility, and always on their sides. Some are found 



in brooks and fianitains, often united in jiuiis, l)at the greater nund)ei- ndiabit the salt water. 

 Thcv are of an unifinm eidour, varying from reddish to green. 



They may l)e c(]niprised in the smgle genus Gaiiiiiiarus, Fab., wdiieh may be distributed into 

 three sections, from the form and number of the legs : — 



1. Those wdiieh have fourteen feet, all of wdiich 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. Tliose which have only ten feet. 



The first of these sections [Homopoda, Westw.] is divisible into two subsections : — 



