10 THE PHYSICAL, KINSHIP 



body cavity — the one corresponding to the visceral 

 cavity of vertebrates — and the main nerve trunk, 

 instead of extending along the back, as among 

 vertebrates, is in invertebrates located ventrally. 

 Vertebrates are the only animals on the earth that 

 have a highly developed circulatory system, a 

 system entirely shut off from the other systems, 

 and containing a heart, arteries, veins, and 

 capillaries. In all invertebrates the digestive and 

 circulatory systems remain to a greater or less 

 extent connected, the blood and food mingling 

 more or less in the general cavity of the body. 

 Worms and insects have pulsating tubes instead of 

 heart and arteries. Crustaceans have hearts with 

 one chamber, and mollusks have two or three cham- 

 bered hearts, but the blood, instead of returning 

 to the heart after its journey through the arteries, 

 passes into the body cavity. In man and other 

 vertebrates the circulating current is confined 

 strictly to the bloodvessels, no particle of it ever 

 escaping into the general body cavity. The heart 

 of vertebrates is distinguished from that of inver- 

 tebrates by being located ventrally. The heart of 

 invertebrates is in the back. The blood of verte- 

 brates differs from that of invertebrates in contain- 

 ing both red and white corpuscles. Invertebrates 

 have white corpuscles only. Worms have yellow, 

 red, or bright green blood. The blood of crusta- 

 ceans is bluish, that of mollusks is white, and that 

 of insects dusky or brown. The blood of all 

 vertebrates, excepting amphioxus, is red. All 

 backboned beings, whether they dwell in seas or 



