xi PHYLUM ARTHROPODA 531 



its inner or mesial border ; two, called exiles (br., fl.), from its outer 

 or lateral Border. The proximal endite (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 gnathobase. The 

 distal endite is rudimentary (en. 6) : the remaining four (en. 25) 

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

 gular, and is called the flabellum (fl.)\ 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 flabellum (fl.) fits 



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



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



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



and the adjacent genital apertures allow of a very convenient 



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



i cephalic segment to that bearing the oostegopods, both inclusive, 



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



[sists of eleven metameres. The remaining segments, from the 



j twelfth to the last inclusive, constitute the abdomen, 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 flabellum 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. cancriformis, 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 

 exoskckton 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 



