ARTHROPODA. 233 



selves successfully to many different kinds of life. None 

 of the phyla are more cosmopolitan than this. They may 

 be internal or external parasites, symbiotic, social, or 

 independent ; they live in water, on the land, burrow in the 

 soil and in other substances as wood and fruits, and fly 

 through the air ; they use all possible kinds of foods ; they 



cf. 



FIG. 79. Diagrammatic cross section of crayfish in the thoracic region, to show 

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

 carapace 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 organs; st, sternal artery; v.a., ventral artery; v.s., ventral blood sinus 

 in which the nerve cord lies. After 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? What is the value of the overarching carapace (c.f.) ? What 

 is the position of the water, in the figure? How is it renewed? 

 See texts. 



