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THE POPULAR SCIENCE MONTHLY. 



which there is a sinus system prolonged from the visceral cavity, but 

 freely communicating with it. In this system the chylaqueous fluid 

 flows alternately in either direction, being propelled by a pump or 

 rudimental heart, which is only a muscular portion of one of these 

 sinuses. The same condition is found in some low crustaceans and 

 arachnids, and in the larvae of certain insects. 



In former articles the sea-urchin has been noticed as the lowest 



possessor of true teeth and stomach, 

 and we now have to award it the 

 added honor of the first distinct 

 heart and blood-vessels. 



The more highly organized in- 

 vertebrates have a muscular heart 

 and true arteries. But the blood 

 always enters the visceral cavity 

 before returning to the heart. In 

 other words, there is no closed cur- 

 rent in the invertebrates, the sys- 

 tem of circulating vessels being 

 in direct communication with the 

 body cavity. Regarding the cir- 

 culation of the lower invertebrates, 

 there is still much uncertainty, as various sets of vessels are found in 

 different groups, the purpose of which is obscure, and their relation to 

 the blood-circulation a matter of investigation. The great variety in 

 the circulation of the many groups of invertebrates renders a detailed 

 description impossible. It will be consonant with the present purpose 

 to briefly describe only a few typical forms. 



The typical system of the articulates is simply a segmented vessel 



Fig. 7. Diagram op SEA-UncmN. a. Anus ; 

 6, Stomach ; c, Mouth ; e, Heart, which by 

 vascular rings encircles the alimentary 

 canal at d and/. 



Fig. 8. Diagram op Articulate Animal, a, TIeart or Blood System ; &, Digestive System; c. 



Nervous System. 



lying lengthwise in the back of the animal. This dorsal tube, trunk, ' 

 or "heart," is open at both ends, and has openings along the sides, 

 guarded by valves. The chylaqueous fluid fills the body cavity, 

 bathing the heart and all the viscera. A puncture of the skin alone 

 allows the blood to issue. The walls of the tubular heart are mus- 

 cular and pulsatile. When the heart expands, the nutritive fluid is 

 drawn in at the hinder end and lateral apertures, and upon contrac- 

 tion it is forced forward and escapes at the forward end, being pre- 

 vented by valves from flowing backward or escaping laterally. The 



