58 



NATURAL HISTORY OF ARTHROPODS. 



while the upper never contains anything but air. On the walls of the upper chamber 

 are found numbers of ramified tufts, containing in their interior an extraordinarily 

 developed network of vascular spaces, thus furnishing a large respiratory surface, 

 while the arrangement of the blood-vessels conclusively proves that blood poor in 

 oxygen enters it from the body, and the vessels leading from it open directly into the 

 auricule. 



In their development the Paguridea hatch as zoese, but from this point they closely 

 resemble the Brachyura in the omission of a Mysis stage. 



SUB-ORDER II. - - BRACHYURA. 



This group, which contains the crabs, the highest of the Decapods, possesses nmch 

 more interest than any other division of the Crustacea, The body is generally broad 

 and flattened, the carapax very seldom exhibiting the cylindrical form so characteristic 

 of the Macrura. The abdomen is relatively small, and is almost always carried flexed 

 under the thorax, which is frequently excavated for its reception. The antennae are 

 small (except in a few forms), and the inner pair frequently can be folded in small 

 grooves or pits in the anterior part of the carapax. The external maxillipeds are oper- 

 cular in character (that is, they form a sort of lid, covering the other mouth-parts) and 

 rarely have but a distant resemblance to the ambulatory feet. The abdomen, as has 

 been said, is small ; the abdominal appendages are very rudimentary, and some of them 



are frequently aborted. In many cases, especially in the males, 

 two or more of the abdominal segments become coalesced, 

 all traces of the sutures being occasionally obliterated. All 

 of the Brachyura, except a few of the land crabs, .leave the 

 egg as zoeas, the carapax being armed with three or, in the 

 higher groups, four long spines. Of these one springs from 

 the front of the carapax and points forwai-ds, one arises on 

 either side and is directed downwards and backwards, while 

 the fourth, when present, is dorsal in position, a portion of the 

 heart extending into it. The appendages at first present are 

 but seven in number, the series ending with the second maxil- 

 liped, while the thoracic members are absent or very rudi- 

 mentary. During the zoea stage the larva increases rapidly 

 in size, the growth being accompanied by frequent castings of 

 the shell, and the development of all of the series of append- 

 ages. Then by a single moult the megalops stage is acquired, 

 a schizopodal stage not intervening, a feature which, as we have seen, is shared by 

 the hermit crabs among the Macrura. The Brachyura may be divided into the 

 following groups : Schizosomi, Teleosomi, Maioidea, Corystoidea, Leucosoidea, Can- 

 croidea, and Grapsoidea. 



The SCHIZOSOMI embraces the brachyuran forms which have the outlets of the 

 ovaries in the bases of the third pair of legs instead of in the adjacent parts of the 

 sternum, as in most Brachyura, and also the last thoracic segment free from the 

 others. In some the abdominal appendages are well developed, in others they are 

 rudimentary. So far as is known the zoeas do not develop a doi-sal spine, but the 

 other spines frequently are greatly elongated, and in the porcelain crabs these spines 

 reach a length of several times that of the body. 



FIG. 69. Zoea of Carcinus, 

 a. Natural size. 



