176 AN INTRODUCTION TO ZOOLOGY 



but they are small flat colourless cells of an elongated oval, almost 

 spindle shape, and comparatively few in number. Circulation is 

 effected by peristaltic contractions of the main trunks, but this is 

 greatly augmented by the action of five, or more rarely six, pairs of 

 enlarged muscular vessels, the so-called hearts. As in vertebrates, 

 the circulation is constant, for the main trunks give off branches 

 which ultimately lead to a capillary network whence the blood is 

 gathered up again by other vessels and returned to the main trunks, 

 thus forming a closed system. In spite of this, however, we cannot 

 employ the terms " arteries " and " veins," as there is no such thing 

 as a central heart, and the vessels are described with reference to the 

 organs they supply ; those bringing blood to the organs are afferent, 

 and those conveying it away again, efferents. There are four main 

 series of longitudinal vessels : (i) The dorsal or supra-intestinal 

 vessel lies in the mid-dorsal line above the alimentary canal, to 

 which it is more or less closely attached, and is the largest trunk in 

 the body. The contractions of its walls drive the blood in it from 

 hinder to front end, and the vessel itself breaks up at both ends into 

 an indefinite series of capillaries. In the region of the intestine 

 this trunk . is, as it were, reinforced by another, the typhlosolar 

 vessel, which runs near the ventral side of the typhlosole. It is not 

 usually considered as a separate vessel, as it is itself somewhat 

 indefinite, often duplicated, and connected with the supra-intestinal 

 by a large number of anastomosing trunks. (2) The second largest 

 trunk is the ventral or sub-intestinal vessel situated in the mid- 

 ventral line under the gut, from which it is suspended by a sort of 

 mesentery formed by a fold of tissue. The blood in it passes from 

 before backwards, and at each end it runs into the same capillary 

 plexus as the dorsal vessel. These two trunks, the sub- and supra- 

 intestinal, are placed in direct communication with one another by 

 means of the five pairs of hearts which are situated in segments 7-11, 

 and sometimes an additional pair in somite 12. As the blood 

 passes forwards in the dorsal vessel it must flow into and fill the 

 hearts, which by their muscular contraction drive it down into the 

 ventral vessel. The direction of flow anterior to these structures is 

 not obvious. (3) Right on the ventral side of the ccelom lies the 

 nerve cord, and attached to the ventral surface of this is the sub-- 

 neural vessel, in which the blood flows from before backwards. 

 In the twelfth or thirteenth segment, and all posterior to them, this 

 vessel is directly connected with the dorsal vessel by a commissural 

 vessel running laterally around the side wall of the coelom. (4) The 

 last two longitudinal trunks are a pair of less important ones lying 

 at the side of the nerve cord, and known, therefore, as the lateral 

 neural vessels. The dorsal vessel in segment 10 gives off a pair 



