138 



1 N \- KltTEBI! ATE AN I J! A LS. 



poseil of .1 c:)iiVL-x u]ii)ei- [il.ite ((') and a flatter ventral arch (s), and 

 each may supiiort a, .single pair cf appendages. Typically, each 

 appendage in a ('ru.stacean con.sists ni a short basal portion or " ])ro- 



topodite " (tig. 80, a), which 

 gives oi'igin to an outer and an 

 inner branch or division, known 

 respectively as the " exopodite " 

 (h) and the " endopodite " (c). 

 Very commonly one of the two 

 divisions of the appendage may 

 be wanting ; or in some cases 

 both are absent, and the ap- 

 penilage may Ije reduced to tlie 

 protijpodite only. A certain 

 mnnber of the appendages of 

 the anterior segments are usu- 

 ally converteil into masticating 

 organs, some of which may be 

 proper biting jaws (mandibles 

 and maxilhe), while others are 

 merely limbs so far modified 

 as to act as subsidiary jaws 

 ("foot-jaws" or maxillipedes). 

 Tlie l;ist segment of the budy— known as the " telson " — carries no 

 appendages, and is often regarded as not being jiroiierly a segment, 

 but an unpaired ;ip]iendage. 



The Criixt<i,-i-(i form an extremely large grouj) of animals divided 

 into many orders, and it will be sufficient to merely notice many of 

 these in the briefest manner. It will also be as well to commence 

 with the consideration of the higher sections of the class first. 



.sr..— Til. 



IC I.nlist 



tliirrl .ilidmriin.il segment nf 

 . / l)(,rsll iivli : s Vriilral 



r tin- sr.^niriil ; ,< l>nit.ilii»liti- ; h 



lit.- ; I- Eiiiliii-odHr. 



ORIiKR JJECAPOIIA. 



Tile C'nixliicfa iiicludeil in this order ilerive their name from the 

 fact that they all pussess five pairs of legs (tir. (A'/'Cf., ten ; pnji-s, 

 feet). They lielong U< n laige section known as the "stalk-eyed" 

 (Crustaceans ( l'(i(h>plithi(lmiitii), from the ftict that tlie eyes are sup- 

 ]i(irteil by long, mov.-ible stalks. Tliey all further htive the cejihalo- 

 tliorax covei-ed by a gi'eat sliielil or " c;ir,i]iace," under which the 

 gills are carried. They inelnde the Lulisters, Rhrim|i.s, t'ray-fisli, 

 <.'iabs, Hermit-crabs, and other forms, and .are the most liighly 

 oiL.niised and most familitir of the whole class of the Cnisfiteea. 

 'J luy an- divided into three very well marked groups or tribes, all 

 of wliieli ,-aii be exemjilified by familiar tyjies. 



