E. A. Andrews — Anatoniy of the Spider Crah. 101 



infoldings of the iiitegument, wliicli project into the body civity as 

 firm, tliin, calcified partitions. 



The partitions, or apodemes, arising from the pleural region unite 

 with those from the stei'nal region and form two tiers of apodenial 

 cells or chambers, communicating with one another and with the 

 central part of the body cavity. For each of the five segments bear- 

 ing ambulatory legs, there are two of these cells, an upper and a lower 

 one, except for the last segment, which has only one. Since the 

 pleural apodemes arise a little behind their corresponding sternal 

 apodemes, each upper cell is a little Ijchind the lower cell of the same 

 segment; and in the first of these segments the lower cell is produced 

 upward to the pleural wall of the body in fi'ont of its corresponding 

 upper cell. The sternal apodemes, being as long as the grooves on 

 the sternal plastron, pass nearer to the center of the body than do 

 the pleural apodemes and terminate on sides of the swollen inner 

 surface of the plastron. The two tiers are separated on the inner 

 aspect by a broad cliitinous band formed by the union of their walls, 

 the posterior wall of the first lower cell being greatly produced back- 

 ward and that of the fifth cell forward over the united intervening 

 partitions. These bands are continuous posteriorly with the shoi't 

 median apodeme, which in the male is indicated by an external 

 groove. In front of the ambulatory legs the apodemes are much 

 smaller, forming, for the posterior three segments bearing mouth 

 appendages, mere frameworks, still with upper and lower openings 

 corresponding to the cells. The apodemes of the two preceding seg- 

 ments form, chiefly, two pairs of flat processes projecting into the body 

 cavity and joined across it by tendinous bands. A pair of similar 

 processes is formed by the apodemes between the second and third 

 of the three segments just mentioned. 



The chitinous covering of the body is lined by the true dermis (<?, 

 fig. l), from which it is secreted, and which also envelopes the 

 apodemes and tendons. Over the dorsal surface and in the legs this 

 skin is beautifully mottled with red, black and yellow. 



The Appendages. — The various appendages of the crab may be 

 distinguished according to their functions : as sense organs, belong- 

 ing to the head ; manducatory organs, belonging to the head and 

 thorax; locomotive organs, belonging to the thorax; and the organs 

 subservient to the reproductive function, which belong to the abdo- 

 men. Of these the last, those of the abdomen, difler greatly in the 

 two sexes. 



The abdomen of the male (fig. 2) bears two pairs of simple, tlu-ee- 



