144 ANNELIDA [CH. 



broken-down de"bris of leaves, etc., which is contained in the earth. 

 The actual minerals of the earth are not digested but are passed 

 out of the body in the form of those coiled and thread-like castings 

 which are so commonly seen on a lawn in the early morning. 

 Earthworms also eat fallen leaves and to this end they drag the 

 leaf-stalks into their burrows, and on autumn mornings it is a 

 common sight to see lawns studded with the stalks of horse- 

 chestnut leaves or the needles of fir trees, the stalks having been 

 dragged a little way into the burrows by the worms. The burrows 

 that they make admit both air and rain to the deeper layers of the 

 soil, and the earth which they swallow in their burrows is brought 

 to the surface and spread about in the form of castings. This is 

 carried on to such an extent that the whole surface of the soil soon 

 becomes covered by a layer of earth brought up from below. It is 

 thus clear that the earthworm is of great use as an agricultural 

 agent. 



All the blood-vessels are remnants of the primary body-cavity 

 which is nearly obliterated as the result of the expansion of the 

 secondary body-cavity or coelom they may be described as being 

 merely crevices between the coelomic wall on the one hand and the 

 ectoderm and endoderm on the other. Those mentioned below 

 are merely the larger channels in a continuous network of spaces. 

 The contractile power which some, like the hearts, dorsal vessel, 

 and sub-intestinal vessel, possess is due to the presence of a special 

 wall of muscular cells derived from that part of the coelomic wall 

 which lies next them. A few of the blood-vessels seem also to 

 have an inner wall consisting of flattened cells this is known as 

 an en dot helium. Its origin is curious and interesting. In the 

 blood of the earthworm as in that of almost all animals there are 

 floating cells which, like those in the coelom, resemble Amoeba in 

 the power of crawling and of emitting pseudopodia; certain of these 

 amoebocytes adhere to the walls of the crevice which constitutes 

 the blood-vessel, become flattened and make in this way an almost 

 complete plating of cells. 



The fluid in the primary body-cavity is the medium into which 

 the digested product of the food diffuses from the endoderm cells, 

 and in which the excreta accumulate until they are removed by the 

 excretory organ. The oxygen necessary for the respiration of the 

 mesenchyme cells must also diffuse into it through ectoderm or 

 endoderm or both. Hence this fluid which is the basis of blood has 

 three main functions : (a) it conveys the products of digestion to 



