\. PHYLUM ARTHROPODA, 531. 



its inner or mesial border J two, called exiles (br.,Jl.), from its outer 

 or lateral border. The proximal enditc (en. 1) is small, and bears 

 Strong spines; in connection with its fellow of the opposite side 

 it is used to seize food-particles and pass them on to the mouth : it 

 is therefore conveniently distinguished as the gncdhobase. The 

 distal enditc is rudimentary (en. 6) : the remaining four (en. 8-6) 

 are long, jointed filaments. The proximal exite is nearly trian- 

 gular, and is called the fiabellum (Jl.); the distal exite is oval, and 

 is known as the bract (br.) ; both probably serve a respiratory 

 function. 



The seventh appendage (7) has only two podomeres in the axis, 

 and the endites are comparatively short and flat. The next eight, 

 i.e. those borne on the third to the sixteenth free segments, closely 

 resemble one another : each (8) has an unjointed axis and short 

 leaf-like endites, the whole appendage having a distinctly foliaceous 

 character. The sixteenth appendage — that of the eleventh free 

 segment — resembles its predecessors in the male, but in the 

 female (9) is peculiarly modified. The distal portion of the axis 

 forms a hemispherical cup, over which the fiabellum (Jl.) fits 

 like a lid : in this way a capsule or brood-pouch is produced, 

 which serves for the reception of the eggs, and the appendage 

 is distinguished as the oostcgopod or brood-foot. The brood-feet 

 and the adjacent genital apertures allow of a very convenient 

 division of the body : all that region from the first free or post- 

 cephalic segment to that bearing the oostegopods, both inclusive, 

 is called the thorax, and its appendages the thoracic feet: it con- 

 sists of eleven metameres. The remaining segments, from the 

 twelfth to the last inclusive, constitute the aldomen, and their 

 appendages are called the abdominal feet. 



The abdominal resemble the thoracic feet in general characters, 

 having the same foliaceous form (10), with unjointed axis, small 

 leaf-like endites, and large fiabellum and bract. They gradually 

 diminish in size from before backwards ; and, from the third abdo- 

 minal segment onwards, two or more pairs of appendages spring 

 from each segment, so that while the total number of abdominal 

 segments, in A. cancrif minis, is twenty-two, and the five hinder- 

 most of these are without appendages, there are altogether fifty- 

 two pairs of abdominal feet. It seemed probable that segments 

 bearing more than one pair of appendages represent two or more 

 fused — or, perhaps one should rather say, imperfectly differentiated 

 — metameres. 



Body-wall. — The whole body is, as already mentioned, covered 

 by a layer of chitin of varying thickness, which constitutes an 

 exoskeleton or external supporting structure. Immediately under- 

 lying it is the deric epithelium or epidermis, from which the chitin 

 is secreted layer by layer. Thus the exoskeleton of Apus is a con- 

 tinous cuticular structure, exhibiting segmentation in virtue of the 



M M 2 



