278 ZOOLOGY. 



(cray-fish and spiders). The Malpighian tubules (see dissec- 

 tion of the grasshopper) associated with the hind gut are 

 believed to be excretory rather than digestive in function. 

 The digestive system as a whole is strikingly correlated with 

 the character of food used, which is exceedingly diversified 

 in this phylum. This can be appreciated only by extended 

 observation and comparison. The student is urged to com- 

 pare such figures of these organs as he may be able to find in 

 the reference texts at his command. 



314. Respiration. In some instances the Arthropoda ob- 

 tain their oxygen directly from the air, in others from the 

 water. In the latter the exchange is effected through the body 

 wall, or by gills. These are essentially thin outgrowths of 

 the body wall, with the cuticula much reduced or absent, 

 into which the blood passes (e. g., the majority of Crustacea). 

 In the former it takes place wholly by means of tubular air- 

 passages or trachea (insects), or these may be supplemented by 

 thinned folds of the body wall book-lungs, (spiders). By 

 these devices the oxygen of the air or water and the blood 

 are brought into intimate relations. In the water-breathing 

 forms the gills are either the modified appendages (Limulus, 

 Asellus}, or specialized outgrowths from them or from the 

 general body wall (cray-fish) (Fig. 124, g). The gills 

 vary widely in number and position, but are found especially 

 in connection with the thoracic and abdominal appendages. 

 The air-breathing forms possess a system of interbranching 

 tubes which may open to the exterior by a pair of stigmata or 

 pores in each somite. These tubes unite, branch and penetrate 

 to every portion of the body. The air is carried to the blood 

 rather than the blood to the air. The tubes are lined by a 

 thin layer of cuticle, and are kept open by a spiral thread of 

 the same material reinforcing the wall. The book-lungs when 

 present lie within a sac which opens to the exterior by a stigma 

 or pore, and consist of a series of plaitings, within which the 

 blood circulates and between which the air circulates. 



The larvae, especially of air breathers, are often developed 



