PHYSIOLOGY OF THE INVERTEBRATA. 189 



traverse the body in a longitudinal direction (Fig. 37, a, b, 

 c, d). 



The dorsal or supra-intestinal vessel is situated on the 

 dorsal side of the alimentary canal. The supra-neural or 

 Bub-intestinal vessel is situated along the ventral side of the 

 alimentary canal ; and the sub-neural vessel lies directly 

 beneath the great ventral ganglionic nerve cord. Besides 

 the three principal vessels, there are two lateral neural 

 vessels situated on either side of the nerve (Fig. 37, b). The 

 dorsal vessel (which is contractile, and consequently drives 

 the blood from behind forward) is connected with the supra- 

 neural vessel in nearly every segment by pairs of transverse 

 vessels — i.e., one vessel on each side of the body connects the 

 dorsal to the ventral trunk. 



In the anterior portion of the body the longitudinal vessels 

 break up into a blood plexus, consequently in this region {i.e., 

 first seven segments) there are no distinct transverse vessels. 

 Between the seventh and tenth segments, the dorsal vessel 

 becomes dilated into what is known as the " hearts " of 

 Ztombricus. These " hearts " contract so as to force the blood 

 from the dorsal to the ventral side of the body. The dorsal 

 vessel also sends out branches to the body wall, mesenteries, 

 and to the walls of the alimentary canal. The supra-neural 

 vessel sends out branches to the nervous system, and also 

 transverse vessels which unite with the sub-neural trunk 

 (Fig. 37, d). Certain transverse vessels also unite the dorsal 

 to the sub-neural vessel ; these vessels supply the segmental 

 organs and integument with blood. 



(b) The body or perivisceral cavity in Hiritdo is only im- 

 perfectly differentiated from the vascular system. It is filled 

 with loose connective tissue in which are dorsal, ventral, and 

 lateral spaces (sinuses) containing blood. 



The vascular system (Fig. 38) consists of a ventral blood- 

 vessel or sinus, and two wide lateral vessels which run along 

 the sides of the body. There is also a median dorsal vessel. 

 All these vessels anastomose with each other, and send off 



