220 



TUB LAWS OF CENTRALIZATION 



cephalothorax becoming a prominent 

 feature and the tail a less conspicuous 

 one; the former gradually extending 

 backwards so as to overlap several of 

 the rings behind it, whilst the latter be- 

 comes by degrees bent under the fore- 

 part. In this condition you will find 

 the lobster; and in the crab, by the 

 still further carrying out of this law, 

 as the most eminent and profound of 

 American naturalists* has shown it to 

 be, the tail is almost entirely obscured, 

 and the cephalothorax covers the entire 

 length and breadth of the body. 



In the next higher class, the Arach- 

 nida, or Scorpions and Spiders, the law 

 of centralization and cephalization (ten- 

 dency toward a head) is carried out in a 

 different form from what appears among 

 Crustacea. Among scorpions the body is divided into two 

 principal regions, a chest, so called, and a tail, but both 

 are elongated and distinctly ringed. In the Spider-group, at 

 least among the highest of them, both the chest 

 (fig. 129, r.r) and tail, or abdomen (t) more prop- 

 erly speaking, are concentrated, and the joints 

 are entirely obliterated. We have thus a dis- 

 linct specialization of two parts of the animal, 

 a division of the body which foreshadows the 

 more eminent Insects; and in still further con- 

 firmation of the tendencies of this group, we 

 find among the highest of them, the Garden 

 Spiders and the like, an initiatory step to sep- 



Fig. 128. 



Fig. 129. 



* See J. D. Dana, in Silliman's American Journal of Science, 1856, vol. xxn. 

 Fig. 128. Cyclops quadricnrnis. Milll. 50 diam. A fresh-water, shrimp- 

 like Crustacean, seen from the back, cr, the cephalothorax ; t, the tail ; f, the 



