TYPE CRUSTACEA. 



369 



principal sense-organs and the organs of mastication (Fig. 

 162), there being behind this head, more or less perfectly dis- 

 tinguishable, a thorax and an abdomen. Judging from the 

 number of pairs of appendages arising from this head region 

 it seems that the typical number of rnetameres consolidated 

 to form it is five, but to these there must be added an anterior 

 segment which does not bear appendages but upon which the 

 eyes are developed. To these six somites there are added, 

 especially in the more highly-differentiated forms, a number 

 of additional metameres which properly belong to the thorax, 

 the apparent extent of the head region being thus increased. 



FIG. 162. A DECAPOD CRUSTACEAN, Cambarus. 



There is indeed throughout the Crustacea a tendency towards 

 what has been called " cephalization," i.e., a condensation of 

 the anterior metameres, and as a rule the higher the form the 

 greater is this condensation and the greater the apparent ex- 

 tent of the head region. The number of segments composing 

 the thorax and abdomen is exceedingly variable in the lower 

 forms, but in the higher there are constantly eight thoracic 

 and seven abdominal segments, the posterior one, termed the 

 telson, being alone destitute of appendages. Frequently, 

 especially in the higher forms (Fig. 162), the thoracic seg- 

 ments consolidate to a greater or less extent, the segmentation 

 of this region of the body being indicated in some forms only 

 by the appendages and the nerve-ganglia, and furthermore 

 lateral folds of the body-wall may project backwards from the 

 sides and dorsum of the head or anterior thoracic regions, 

 enclosing the thorax or even the entire body in a firm cara- 

 pace or else in a bivalved shell, sometimes provided with 

 adductor muscles. 



