ARTHROPODA 289 



the septa contain numerous capillaries, so that the whole 

 organ has been compared with the glomerulus of the Vertebrate 

 kidney, and doubtless subserves the same functions. It is 

 by no means certain that the above-mentioned sac is coelomic 

 in nature, and in his last paper Weldon is inclined rather to 

 regard it as an enlarged portion of a nephridial system, such as 

 exists in Mysis, and not as a remnant of a primitive coelom. 



The history of the developement of Falaevionetes varians, 

 recently described by Allen, throws some light upon the 

 nature of the body-cavity in this group. The body-cavity of 

 this animal consists of four regions : (i.) a dorsal sac in which 

 the cephalic aorta lies, (ii.) a central cavity containing the 

 liver, intestine, and nerve cord, (iii.) two lateral cavities con- 

 taining the proximal ends of the shell glands, and (iv.) the 

 cavities of the limbs containing the distal ends of the same 

 glands. Of these, the first, or dorsal sac, is truly coelomic, its 

 cavity being homologous with that of the dorsal portions of the 

 mesoblastic somites of Peripatus (vide p. 308). The central 

 and lateral cavities, and those of the legs, represent a pseudocoel. 

 The nephro-peritoneal sac described by Weldon arises in this 

 genus from an enlargement of the tube of the green gland. 



In the long-tailed Decapods it is usual to find six pairs 

 of abdominal ganglia, but in Pagurus, the hermit-crab, there 

 is only one large one. In these animals the abdomen is soft and 

 distorted into the shape of the interior of the Mollusc's shell 

 which affords it shelter; the abdominal limbs are usually 

 rudimentary, but the chelae or first pair of thoracic limbs are 

 large, and can be closed down over the mouth of the shell, 

 acting as a kind of operculum. 



In the Brachyura a great concentration of the nervous 

 system has taken place, and the ventral ganglia have all fused 

 together into a common oval mass. The Decapods, in addition 

 to the main chain of ganglia, possess a very extensive and 

 complex system of visceral nerves. 



Besides the olfactory hairs on the first pair of antennae, 

 and the compound stalked eyes, it is characteristic of the 

 Decapods to possess auditory organs, situated at the base of 

 the antennules. These take the form of hollow sacs open to 

 the exterior; their walls support auditory hairs connected 



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