ARRANGEMENT OF TISSUES. 157 



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

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

 (/land-cells, have the power of producing within their substance 

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

 pores in the cuticle. The mucus often accumulates within these 

 cells to such an extent as to swell them up, giving them an oval 

 form and clear appearance. Many of the cells are branched at 

 their inner ends. 



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

 running around the body. On the upper side they are inter- 

 mingled with connective-tissue cells containing a granular brown- 

 ish substance (pigment) which gives the dorsal aspect its darker tint, 



4. Longitudinal Muscles (l.m}. A layer of muscle-fibres run- 

 ning lengthwise of the body. They are arranged in complicated 

 bundles, which in cross-sections have a feathery appearance. 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. Peritoneal Epithelium (p.e). A very thin and delicate 

 layer of flattened cells attached by their edges and covering the 

 inner face of the muscular layers. This is only visible in very 

 good sections. It extends over the whole surface of the ccelom, 

 covering also the organs which the ccelom contains. 



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 ccelom. The typhlosole (ty) is seen to be a deep 

 infolding of its upper portion. The wall is composed of six 

 layers as follows, starting from the alimentary cavity : 



1. Cuticle (<?, Fig. 88). A transparent structureless mem- 

 brane like the outer cuticle, with which it is continuous through 

 the mouth and anus. In the gizzard it is enormously tough and 

 thick, and is roughened by minute points. 



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

 row, columnar cells with oval nuclei. 



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



