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 larvæ of certain insects.
In former articles the sea-urchin has been noticed as the lowest 
Fig. 7.—Diagram of Sea-Urchin. a, Anus; b, Stomach; c, Mouth; e, Heart, which by vascular rings encircles the alimentary canal at d and f. 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 invertebrates 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 current in the invertebrates, the system of circulating vessels being in direct communication with the body cavity. Regarding the circulation 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. 8.—Diagram of Articulate Animal, a, Heart or Blood System; b, 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 muscular and pulsatile. When the heart expands, the nutritive fluid is drawn in at the hinder end and lateral apertures, and upon contraction it is forced forward and escapes at the forward end, being prevented by valves from flowing backward or escaping laterally. The
