ARTHROPODA. 



279 



in conditions very different from those chosen by the adults. 

 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). 



FIG. 124. 



FIG. 124. Diagrammatic cross-section of Cray-fish in the thoracic region, to show 

 relation of circulation and respiration, a, appendage; c, carapace; c.f., flap of cara- 

 pace overhanging the gills; d, digestive tube; g, gill; h, heart; /, liver; m, body muscles; 

 m', muscles of the appendages; n.c., nerve cord; p.s., pericardial sinus; r, reproductive 

 glands; st, sternal artery; v.a., ventral artery; v.s., 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 arteries and 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 ? 



315. Circulation. The heart or pulsating organ when 

 present is dorsal and may be much elongated, with an enlarge- 

 ment in each somite. It lies in a membrane-bounded cavity 

 called the pericardial sinus (Fig. 124, ps."), which is a part of 

 the haemoccele or secondary body cavity ( 310). The blood 

 comes to the pericardial cavity and enters the heart by means of 



