fluid finds its way backward through the lacunae or passages between the tissues and viscera. The dorsal vessel prevents the stagnation of the fluid.
In the myriapods the dorsal trunk has as many segments as there are joints of the body. One of the millepeds has not less than one hundred and sixty. Centipeds have generally twenty-one segments, and besides the pair of valves for each joint there is given off a pair of arteries. These unite to form a ventral tube. Insects have the heart segmented only in the abdomen, and never more than eight segments. An arterial prolongation of the trunk as a simple tube extends to the head.
As spiders and scorpions have localized breathing-sacs, the require 
Fig. 9.—Diagram of Mollusk. a, Alimentary Canal; h, Heart; n, n’, n", Nervous Ganglia. a respiratory circulation. This is secured, not by special tubes, but by the passage of the blood, on its return to the heart, through venous sinuses or special passages between the internal organs.
The best heart among articulates is possessed by the crustaceans, the largest, though not the highest, animals of the sub-kingdom. Crabs and lobsters have a concentrated heart, a short, fleshy sac, with great propelling power, which sends the blood by several branching arteries to the parts of the body. We now find a concentration of the power which had been previously diffused in a long tube.
Fig. 10.—Cross-sectional Diagram of a Fresh-water Mussel. f, Ventricle; g. Auricles; c, Rectum; p, Pericardium; h, i, Gills; B, Foot; A, A, Mantle or Skin.
As the mollusks are mostly so sluggish that their circulation has little aid from the movements of the body, they require a more pow-
