82 ESSENTIALS OF ZOOLOGY 



layers of the septa, but it becomes prominent in the blood 

 vessels and the blood. 



Vascular System. The blood is formed of a plasma coloured 

 by haemoglobin and containing colourless leucocytes. Haemo- 

 globin is a complex proteid substance united with iron, and is 

 a respiratory pigment. It has the power of uniting loosely 

 with oxygen, and thus acts as a carrier of the oxygen to all 

 parts of the body of the worm, where it is given up to the 

 organs and tissues, or rather, the cells composing these. 

 Breathing thus takes place in a complex body by the blood 

 conveying the oxygen, in this case from the ectoderm to the 

 most remote parts of the body. The blood as it circulates in 

 the walls of the intestine takes up food material, and this like- 

 wise is conveyed to regions where it* can be employed for pro- 

 viding energy or for building up and growth. The blood also 

 serves to convey waste material from the cells of the body in 

 general to the excretory organs. The blood is contained in 

 vessels, the larger of which are plainly seen. The alimentary 

 canal is traversed dorsally by a dorsal vessel. In this vessel, 

 which is contractile, the blood runs forwards and the vessel 

 ends anteriorly in branches over the pharynx and buccal sac. 

 During its course it receives vessels from the intestine. It is 

 connected around the oesophagus by five pairs of contractile 

 vessels, called hearts, with a ventral vessel which lies under the 

 alimentary canal. An oesophageal vessel is given off also in 

 the same region, which distributes blood to the lateral walls of 

 the oesophagus and pharynx. The ventral vessel is connected 

 by segmental branches with vessels lying one on each side of 

 the nerve cord, and consequently called lateral neurals, and 

 with a subneural vessel which lies medianly under the nerve 

 cord, and it also supplies the intestine. These are the collect- 

 ing vessels, and the blood they contain is sent mainly to the 

 nephridia and to the skin. The afferent nephridial vessel 

 passes from the ventral in each segment, and the efferent 

 nephridial vessel enters the parietal. The parietal vessels, 

 one in each segment on each side, pass up the outer wall of the 

 coelom and spread into spaces underneath the ectoderm and 

 into intercellular spaces. The skin is kept moist by the action 

 of the gland cells and by exudation from the dorsal pores, 



