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 M/ysis, and not as a remnant of a primitive coelom. 
The history of the developement of Palaemonetes varians, 
recently described by Allen, throws some leght 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 Mollusce’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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