xr PHYLUM ARTHROPODA 519 



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 brood-pouch is produced, which 

 serves for the reception of the eggs, and the appendage is dis- 

 tinguished as the oostegopod 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 postcephalic 

 segment to that bearing the oostegopods, both inclusive, is called 

 the thorax, and its appendages the thoracic feet : it consists of 

 eleven metameres. The remaining segments, from the 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 seems 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 

 cxoskeleton 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- 

 tinuous cuticular structure, exhibiting segmentation in virtue of the 

 fact that, while comparatively thick and strong in places where 

 no movement is required, it is thin and flexible in the intervening 

 spaces, and thus allows of the movement of the harder parts upon 

 one another. 



The seta3, which occur on many parts of the body, and in parti- 

 cular fringe the appendages, are hollow offshoots of the chitinous 

 cuticle, containing a protoplasmic core continuous with the epi- 

 dermis. They thus differ fundamentally from the seta3 of 

 Chaetopods, which are solid rods sunk in muscular sacs. 



The muscular system is well developed (Fig. 431). Under- 

 lying the epidermis is a layer of connective-tissue, and beneath 

 this is found, in the posterior or limbless part of the abdomen, a 

 layer of longitudinal muscles encircling the body and attached by 

 connective-tissue to each segment. In this way the muscular layer 

 is itself segmented, being divided by the connective-tissue insertions 

 into muscle-segments or myomeres. The action of these muscles is 

 to approximate adjacent segments : according as the fibres on the 



