Chap, xv.] KESPIEATORY OEGANS IN ANIMAL KINGDOM. 233 



(Fig. 26). Analogous systems exist in many other echinoderms, 

 and often they are bedecked with vibratile cilia.^ 



The crustaceans breathe through the branchise. which often are 

 external appendices. Sometimes 

 they are abdominal appendices, 

 sometimes they are claws, or por- 

 tions of claws modified. In the 

 decapods we find branchise inclosed 

 in cavities furnished with eddying 

 organs, making the water cir- 

 culate. 



In a certain number of arach- 

 nida, in the myriapods, and espe- 

 cially in the insects, respiration 

 is accomplished by means of the 

 tracheae, which the nutritive liquid 

 of the animal bathes exteriorly 

 (Kg. 28). The tracheae form 

 then a system of canals, whose 

 ultimate ramifications constitute 

 a fine network, analogous, as to 

 arrangement, to the capillary net- 

 works.^ The principal trunks are 

 kept open by a chitineous bande- 

 lette rolled spirally and following 

 the skeleton of the tracheae. These 

 tracheae communicate outwards by 

 means of determinate orifices 

 called stigmata. The introduc- 

 tion and the expulsion of the air of the tracheae seem 

 to be helped by regular movements of the abdominal 

 walls. These rhythmical movements are frequent; we can 

 count on an average twenty-five in the stag-beetle, and from 



' Dugis, loc. cit., t. II. p. 355. 

 ^ Leydig, loc. eit., p. 440. 



Fio. 27. 



A, transversal section of a phyllopoil 

 (Limnetis), The section passes through 

 the part which bears the first pair of 

 feet.— ^ intestiual canal ; c, heart ; 

 % ventral marrow ; d, folding of the 

 teguments forming a shell conceal- 

 ing the members ; &r, natatory feet. 

 — B, transversal section of sgualla 

 (through the abdomen), i^ c, n, as in 

 A ; m, muscles ; d, tegumentaiy fold ; 

 p, external lamellary parts ; j/, inter- 

 nal lamellary parts ; br, branchicB, 



