86 MANUAL OF ANIMAL BIOLOGY 



intestine is roughly circular in outline with a large central cavity. 

 The latter is partly filled by an infolding of the dorsal wall to form 

 the typhlosole, which greatly increases the effective absorptive 

 surface. The outer surface of the intestine and the interior of the 

 typhlosole is covered by layers of long, cylindrical cells, composing 

 the chloragogen layer. These cells are believed to have an 

 excretory function, and the yellowish green pigment granules 

 scattered through their cytoplasm are regarded as metabolic wastes 

 which they have removed from the blood. The mass of chlorago- 

 gen cells continues into the typhlosole and, indeed, forms the 

 greater portion of this structure. (W. f. 61.) 



Lying just within the chloragogen tissue is a layer of circular 

 muscle tissue which forms a band running entirely around the in- 

 testine. Just as in the body wall, the contraction of the muscle 

 fibers of this layer brings about a reduction in the diameter of the 

 alimentary tract. Numerous bundles of muscle fibers, running 

 lengthwise of the alimentary canal, lie below the circular muscular 

 layer. They do not form so definite a layer as the circular muscles, 

 since they are arranged somewhat irregularly. Numerous blood 

 vessels are present in the muscle tissue of the intestine and, be- 

 tween the two layers, blood spaces are found in which the blood 

 receives the absorbed food materials. 



The innermost layer of the alimentary canal, known as the 

 lining epithelium, is composed of endoderm cells which are re- 

 sponsible primarily both for the digestion of the food material and 

 its absorption after it has been digested. There are, in fact, two 

 types of these endoderm cells : the gland cells and the absorptive 

 cells. The gland cells secrete digestive ferments, or enzymes, 

 which are comparable to those present in other animal forms, and 

 these have the power to change, or digest, the food so that it can be 

 absorbed. The gland cells vary considerably in size and shape, 

 depending upon the amount of material they contain. The absorp- 

 tive cells are quite regularly arranged between the gland cells, and 

 are adapted for absorbing the digested food material from the 

 cavity of the intestine and transferring it to the blood stream for 

 transportation to all parts of the body. The bodies of the absorp- 

 tive cells are quite large, and the free ends are ciliated. 



Functional. The Earthworm, as its name indicates, is adapted 

 for living in the soil. The holes, or burrows, which it makes, vary 

 in depth from a few inches to several feet, depending upon the 



