The laws and mechanics of circulation, with the principle involved in animal movement . an accompanying lateralbranch is seen, as usual, on both sides of the same columns. Anterior to the commencement of the stomach, the greatdorsal artery (Fig. 153, b, b) communicates with the mediansubgastric vein (a, a) by five or more pairs of lateral, wide, sac-culated arches (c, d), which embrace the oesophagus, as thecorresponding vascular arches which connect these t wo vesselsin other annelides and in the entomoid classes. Accordingto Dr. Williams, the following is the plan of circulation inearth-worm
The laws and mechanics of circulation, with the principle involved in animal movement . an accompanying lateralbranch is seen, as usual, on both sides of the same columns. Anterior to the commencement of the stomach, the greatdorsal artery (Fig. 153, b, b) communicates with the mediansubgastric vein (a, a) by five or more pairs of lateral, wide, sac-culated arches (c, d), which embrace the oesophagus, as thecorresponding vascular arches which connect these t wo vesselsin other annelides and in the entomoid classes. Accordingto Dr. Williams, the following is the plan of circulation inearth-worms (Fig. 154): The circulation is very complicate. NO HEART IN THE LEECH. 383 They respire through the cutaneous surface, which is occu-pied by a dense capillary plexus ; also through the alimentarycanal, which is similarly supplied, and which undoubtedlyfunctions as a respiratory as well as digestive organ. It is alsoclaimed that the stratum of viscid matter in which they arealways enveloped is remarkably endowed with the property ofabsorbing and dissolving atmospheric air (Jones).. Eig. 154.—Diagram. Illustrative of the Circulatory Apparatus in the Leech (HirudoMedici>.alis).—After Dr. Williams, a, Great dorsal vessel ; c, ventral vessel; d, d,intercommunicating vessels between dorsal and ventral trunks ; e, e, lateral abdomi-nal trunks ; g, vessels distributed over the cascal appendages to the stomach. From the equal development of the great vascular trunksand the numerous transverse anastomoses, it is easy to per-ceive how the circulation in these animals can, by the closingof the divided ends of the vessels, become accommodated toextensive mutilations, and proceed without interruption in afew segments detached from the trunk. Some of the simplerforms of annelides, e. g., styluria, are thus enabled to ex- 384 NO HEART IN- THE LEECH. tend their means of propagation by the spontaneous transversefission of the body. But the leech, perhaps, presents the most complicate va
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