ARTHROPODA 



283 



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 spiracle or pore, and consist of a series of pleat- 

 ings, within which the blood circulates and between which the 

 air circulates. 



The larvffi, especially of air breathers, are often developed 

 in conditions very different from those chosen by the adults. 



Pig. 126. 



t— . 



Fig. 126. Diagrammatic cross section of Crayfish in the thoracic region, to show relation 

 of circulation and respiration, a, appendage; c, carapace; c.f., flap of carapace overhanging the 

 tfills; dt digestive tube; £. gill; h, heart; 2, liver; m. body muscles; m', muscles of the appendages; 

 n.c, nerve cord; fi.s., pericardial sinus; r, reproductive glands; st, sternal artery; v.a,, ventral artery; 

 v.s.t ventral blood sinus in which the nerve cord lies. Modified, from Lang. 



Questions on the figure. — What is the relation of the gills to the body wall? 

 Follow the course of the circulation by the arrows. It leaves the heart by definite 

 arteriesand comes back by less definite blood sinuses. What is the function of 

 the valves? What gain is there in the position of the ventral nerve cord in the 

 blood sinus? 



This fact may make necessary very important changes in the 

 respiratory organs in the metamorphosis. Some forms are 

 even water breathers in the larval stage and air breathers in 

 the adult (dragon-flies). 



