ANNULOSA: CRUSTACEA. 



299 



cephalic shield, and is called the "glabella;" it protected the region of 

 the stomach, and is usually divided into from three to four lobes by lateral 

 grooves. At each side of the glabella, and continuous with it, is a small 

 semicircular area, called the " fixed cheek." The glabella, with the " fixed 



C 



Fig. 149. Transverse section of the thorax of Calymene senaria, partially restored 

 (after C. D. Walcott). a Dorsal crust ; b Visceral cavity, continued laterally to the 

 pleural margins of the dorsal crust; c Legs, restored; d Epipodite; e Spiral gills. 

 Enlarged six times. 



cheeks," is separated from the lateral portions of the cephalic shield 

 termed the "movable" or "free cheeks" by a peculiar suture or line 

 of division, which is known as the "facial suture," and is quite unknown 

 amongst recent Crustacea^ except for a faint indication in the Litnulus^ and 

 more or less doubtful traces in certain other forms. The movable cheeks 

 bear the eyes, which are generally crescentic or reniform in shape, are 

 rarely pedunculated (being never supported upon movable foot-stalks), 

 and consist of an aggregation of facets covered by a thin cornea. The 

 facial sutures may join one another in front of the glabella in which case 

 the free cheeks will form a single piece ; or they may cut the anterior mar- 

 gin of the shield separately in which case the free cheeks will be discon- 

 tinuous. The posterior angles of the free cheeks are often produced into 

 long spines. 



Behind the cephalic shield comes the thorax, composed of a variable 

 number of segments, which are not soldered together, but are capable of 

 free motion upon one another, so as to allow the animal, in many cases, to 

 roll itself up after the manner of a wood-louse or hedgehog. The thorax 

 is usually strongly trilobed, and each thorax-ring shows the same triloba- 

 tion, being composed of a central, more or less strongly convex, portion, 

 called the "axis," and of two flatter side-lobes, called the "pleurae." 



The " pygidium," or " tail," is usually trilobed also, and, like the 

 thorax, consists of a median axis and of a marginal limb, the composition 

 of the whole out of anchylosed segments being shown by the existence of 

 axial and pleural grooves. 



ORDER IV. MEROSTOMATA. The members of this order 

 are Crustacea, often of gigantic size, in which the mouth is fur- 

 nished with mandibles and maxillae, the terminations of which 

 become walking or swimming feet and organs of prehension. 



