278 COMPAEATIVE AJ^ATOMY. 



body, just as in the liiglier Vermes ; it is found in all the Arthro- 

 poda. There are no signs in the Arthropoda of the dissepiments 

 found in the Annulata in correlation with the metamerism of their 

 bodies. This shows that their relationship to the Annulata is, to say 

 the least, a very distant one. In all cases the cojlom forms a portion 

 of the blood-vascular system, so that the perienteric fluid found in 

 many Vermes as a fluid different from the blood is represented in the 

 Arthropoda by the blood itself. 



In most Arthropoda some of the form-elements of the mesoderm, 

 which are not applied to the ectoderm — to form the wall of the 

 body — or to the endoderm to form the wall of the enteron — persist as 

 a number of cells, which are not apportioned to any definite oi'gan. 

 These masses of cells remain in various parts of the coelom, and are 

 often found, just like other connective substance in the Arthropoda, 

 between the separate organs embedded in the coelom. 



Sometimes all of these cells remain indifferent, and form, by 

 uniting with one another, cords or networks. As a rule, however, 

 they are differentiated. Drops of fat are formed in them, which 

 either fill up the cells in a regular manner, or run together into larger 

 drops ; in consequence of this these cells are comprised under the 

 name of fat-bodies. This fat is sometimes variously coloured 

 (yellow or red). Cells of this kind, containing fat-drops, have been 

 observed in the Crustacea, and especially in the Entomostraca, 

 where they are sometimes very large in comparison with the size of 

 the aniraaFs body, in which they are distributed in a constant and 

 regular manner. This justifies us in supposing that they have some 

 hydrostatic significance. 



Deposits of this kind are most largely developed in the Insecta, 

 where the fat-body, especially in the larval stages, consists of 

 large cells, connected to one another by processes, and filling up a 

 large part of the coelom. We must not suppose, however, that 

 these cells contain nothing but fat. This tissue is that in which the 

 material, which is partly used up in the pupa stage, is deposited ; in 

 consequence of this not much of it is found in the adult Insect. The 

 cells vary greatly in their mode of connection. This may be close, 

 so that the fat-body forms lamellte, or connected lobes, which are 

 connected with branches of the tracheal system; or the cells are 

 loosely connected together. In the most extreme case the cells may 

 lie separately in the coelom, where they must not be confounded 

 with the much smaller and more indifferent blood-cells. 



The cells of the fat-body in the Tracheata also serve for the 

 deposition of excretory matters, which are known to be urates. 

 These form crystalline concretions, or larger spheres of the form of 

 small granules, which call to mind the renal concretions of the 

 MoUusca. Among the Arachnida they are found in the Mites ; they 

 are found in the Myriapoda (Julus, Polydesmus, Glomeris), and are 

 very common in the Insecta. They do not seem to be altogether 

 absent from the Crustacea, for similar concretions have been observed 

 in Asellus. 



