300 THE EARTHWORM AND OTHER ANMLATA 



cellular covering of the outer surface, perforated for the openings 

 of gland cells, nephridia, sense-organs, setae, and dorsal pores, so 

 that the number and location of these structures can be deter- 

 mined from surface views when the cuticle is stripped from the 

 body. The epidermis is a layer of columnar epithelial cells, among 

 which are gland cells that secrete the mucus which normally 

 covers the body. Sense-organs appear as groups of modified 

 epithelial cells, the inner ends of which are prolonged as the 

 sensory axons that enter the ventral nerve cord (cf. Fig. 144). 

 The circular muscle layer, by which the elongation of the worm's 

 body is produced, lies immediately beneath the epidermis. The 

 cells of this layer are simple non-striated muscle cells, among 

 which there are capillaries of the circulatory system and connective 

 tissue. The longitudinal muscle laijer has a feathery appearance 

 in cross section, because the non-striated fibers are somewhat 

 flattened and attached on either side of sheets of connective 

 tissue that divide the layer longitudinally. Lining the ccelome 

 is the -peritoneum, which is a layer of simple squamous epithelium. 

 Within the ccelome are found the nephridia with their cellular 

 organization and covering of peritoneum {cf. p. 296). The caelomic 

 fluid, which completely fills the body cavity and contains numerous 

 amoeboid corpuscles, may be compared with the lymph in the 

 larger spaces found in the bodies of higher animals. 



The intestine is covered externally with a layer of modified 

 peritoneal cells, which are known as the chloragogue cells 

 and which have been supposed to possess functions resembling 

 in part those of liver cells in vertebrate animals. They are col- 

 umnar and expanded at their outer ends. The center of the 

 typhlosole is occupied by a mass of these chloragogue cells. Beneath 

 this outermost layer of the intestine is a connective tissue layer 

 containing longitudinal and transverse muscle fibers. This layer 

 has a rich supply of capillaries at the bases of the elongated 

 columnar epithelial cells that compose the innermost layer, or 

 mucous membrane, the inner surface of which is covered by a 

 delicate cuticle. 



The structure of the larger blood vessels is such that a wall of 

 contractile and connective tissue cells encloses the blood plasma 

 containing the haemoglobin and numerous blood cells or corpus- 

 cles. The ventral nerve cord (cf. Fig. 144), when seen in such a 

 section, shows nerve cells and their processes. Dorsally there 



