92 THE BIOLOGY OF AN ANIMAL. 



2. Hypodermis (hy) (epidermis or skin). A layer of colum- 

 nar epithelium, composed of several kinds of elongated cells, set 

 vertically to the surface of the body. Some of these, known as 



t/ i/ 



gland-cells, have the power of producing within their substance 

 a glairy fluid (mucus), which exudes to the exterior through the 

 pores in the cuticle. Others (sensory cells) give off from their 

 inner ends nerve-fibres which may be traced inwards to the 

 ganglia (Fig. 43). 



The Clitellum is produced by an enormous thickening of the hypoder 

 mis, caused especially by a great development of the gland-cells. Three 

 forms of these may be distinguished, which probably produce different 

 secretions. The tissue is permeated by numerous minute blood-vessels 

 which ramify between the cells. 



3. Circular Muscles (e.m). A layer of parallel muscle- 

 fibres running around the body. On the upper side they are 

 intermingled with connective-tissue cells containing a granular 

 brownish substance (pigment) which gives to the dorsal aspect 

 its darker tint. 



4. Longitudinal Muscles (Lm). A layer of muscle-fibres 

 running lengthwise of the body. They are arranged in compli- 

 cated bundles, which in cross-sections have a feathery appear- 

 ance. In longitudinal sections they appear as a simple layer, and 

 resemble the circular fibres as seen in the cross-section. 



The circular muscles are arranged in somewhat similar bun- 

 dles, as may be seen in longitudinal sections. 



5. Ccelomic or Peritoneal Epithelium (p.e.~). A very thin 

 layer of flattened cells next the coelomic cavity. 



The hypodermis, and therefore also the cuticle to which it 

 gives rise, is derived from the ectoblast. The other layers (3, 

 4, 5) arise from the somatic layer of the mesoblast. 

 B. ALIMENTARY CANAL. 



The wall of this tube appears in cross-section as a ring sur- 

 rounded by the coelom. The typhlosole (ty) is seen to be a deep 

 infolding of its upper portion. In the middle region the wall is 

 composed of five layers as follows, starting from the alimentary 

 cavity (Fig. 40) : 



1. Lining Epithelium (ep). A layer of closely packed, nar- 

 row ciliated columnar cells with oval nuclei. 



2. Vascular Layer (v.l). Numerous minute blood-vessels. 



