CRUSTACEA. 



163 



of facets, covered by a thin cornea. They are generally cres- 

 centic or reniform in shape, and are invariably sessile, in the 

 sense that they are never supported upon movable stalks. In 

 some cases, however, they are carried upon longer or shorter 

 prominences. The eyes differ much in size, and they are 

 wanting in a few forms, such as the little Agnostt. 



Fig. 105. Bronteus hinatus. 



Fig. 106. Cheirurus pleur- 

 exanthemus. 



Fig. 107. Calymene 

 B lumeiibachii. 



Behind the cephalic shield comes the thorax, composed of 

 a variable number of segments which are not soldered to- 

 gether, but are capable of more or less movement upon each 

 other. The amount of movement thus allowed varies, but in 

 several genera (e.g., Calymene and Jllcemis) it was sufficiently 

 great to allow of the animal completely rolling itself up after 

 the manner of a hedgehog. The number of body-rings or 

 segments in the thorax varies from no more than two (Agnos- 

 tus\ to as many as twenty-six (Harpes ungula). Ordinarily the 

 thorax (fig. 108) is strongly trilobed, and each body- ring ex- 

 hibits the same trilobation, being composed of a central, more 

 or less convex portion, called the " axis," and of two flatter 

 side-lobes, termed the " pleurae." The pleurae are in one piece 

 with the axis, but are separated from it by a more or less pro- 

 nounced groove, the "axal furrow." Each pleura is grooved 

 longitudinally by a deep sulcus, and in many genera the ends 

 of the pleurae are furnished with facets, which have smooth 

 surfaces, and slide under the preceding pleura, in the act of 

 rolling up. In some genera, as in Illcenus (fig. 109), the axis 

 is very broad, the axal furrows are not marked, and the trilo- 



