ANNELIDA. 59 



The undigested remnants are passed out from the anus as 

 cylindrical "worm-castings." These form little heaps on the 

 surface of the ground, and as they are composed of earth brought 

 from below, a mixing of soil is gradually effected. 



The intestine, partly owing to the presence of the typhlosole, 

 offers a large absorbent surface to the digested materials. The 

 products of digestion readily pass into the blood-vessels ramifying 

 in the intestinal wall, and are thence distributed to the body at large, 



4. Circulatory Organs (Fig. 1 8, A). These place the different 

 parts of the body in communication as regards oxygen and the 

 products of digestion and katabolism. They may be divided into 

 (1) Blood System, (2) Ccelom and Ccelomic Fluid. 



(1) Blood System. This is made up of a closed series of 

 tubes containing bright red blood, in which are contained minute 

 colourless nucleated cells, the Hood-corpuscles. The red fluid in 

 which these are suspended is known as plasma. The blood, after 

 death, or on removal from the body, coagulates i.e., sets into a 

 jelly-like mass. There are five chief longitudinal vessels, a large 

 number of transverse vessels, and delicate capillary networks 

 which connect up the various branches. 



Eunning in the median line on the dorsal side of he alimentary 

 canal, and visible through the skin in a fresh specimen, is the 

 dorsal vessel (d.v). This breaks up in front into a network of 

 small vessels on the pharynx. A similar ventral vessel runs 

 longitudinally below the alimentary canal. A longitudinal sub- 

 neural vessel runs below the ventral nerve-cord, and a small lateral 

 neural vessel on each side of it. These longitudinal trunks are 

 connected by transverse lateral vessels, of which the most important 

 are the hearts. These are six pairs of large lateral vessels in 

 segments 6-11, which connect the dorsal and ventral trunks, 

 and are closely attached to the anterior faces of the septa. The 

 typical arrangement of the transverse trunks in a segment of 

 the intestinal region is as follows : (a) A pair of parietal 

 (commissural) vessels, receiving numerous twigs from the body- 

 wall, connect the dorsal and sub-neural trunks; (b) The intestine 

 is supplied by numerous afferent branches from the ventral, 

 which break up into a network, from which the blood returns 

 by two efferent branches to the dorsal; (c) Each nephridium 

 possesses a very rich plexus of vessels, supplied by a branch 

 from the ventral and sending one to the corresponding parietal. 



