CRUSTACEA. 



493 



tLV 



outwards only, into a plate which is known as the " pleuron " or 



"pleura" (fig. S53)- Each segment of the body may carry a pair 



of appendages (even the telson being sometimes furnished with 



such) : and the typical appendage of the Crustacea consists of an 



undivided basal portion, or " protopodite," 



to which are attached two diverging branches 



(fig. 353). The outer of these branches is 



termed the " exopodite," and the inner the 



" endopodite " ; but one or other, or both, 



of these terminal divisions of the appendage 



may be suppressed. 



The head in the Crustacea as a rule 

 carries in front a pair of eyes, which in 

 the higher forms are " compound," being 

 made up of a variable number of separate 

 lenses united side by side. The eyes may 

 be fixed directly to the head, as in the 

 "Sessile-eyed" Crustaceans {Hedriophthal- 

 wata), or may be carried upon longer or 

 shorter movable peduncles, as in the " Stalk- 

 eyed " Crustaceans (Podoflhthal??iata). Be- 

 hind the eyes, the head carries, in general, 

 two pairs of jointed feelers or "antennas," 

 the first pair (" antennules ") being com- 

 paratively small, while the hinder pair 

 (" great antennae ") is of larger size. The 

 segments immediately posterior to this carry 

 three pairs of jaws — the " mandibles," and 

 the first and second pairs of "maxillae." 

 All these are modified appendages, and 

 therefore are in pairs, and work from side 

 to side. The mandibles constitute the most 

 powerful pair of jaws, and consist usually 

 of a strong toothed protopodite to which a Fi°-. 352 —Lobster with ail 

 short endopodite (the " mandibular palp ") is ^T^^S^^Z 

 attached. Between the bases of the mandi- and the abdominal somites 



ii-i r i ii ii separated from one another. 



bleS IS the aperture 01 the mOUth, bounded r, "Rostrum"; ca, Carapace, 



in front by an undivided chitinous plate (the ad'lelmea^f %?£ The 

 "hvpostome" or "labrum"), and behind by ^ rst six segments of the ab- 



.._,,. ' domen. >« o. 6 carries the last 



a USUally forked lower lip Or " metaStOma." pairofswimmerets. /, Telson. 



The first pairs of thoracic appendages are 



generally intermediate in structure between walking-legs and jaws, 

 and constitute what are known as " foot-jaws " or " maxillipedes " ; 

 while the hinder ones are more especially devoted to locomotion. 

 The thoracic appendages vary, however, extremely in form and 



