GENERAL ZOOLOGY 



Dorsal blood vessel 



Typhlosole 



Longitudinal muscle layer 

 Circular muscle layer 



Peritoneum 



Cuticle 



Chloragogue 

 layer 



Setae 



Coelom 



Gut cavity 



Ventral 

 blood vessel 



Lateral neural 

 blood vessel 



Seta 

 Nerve cord 



Fig. 14.14. Dias;rammatic cross section of the earthworm, throusjh the ree;ion of the stomach- 

 intestine. As shown here, such a section contains only irregular portions of septa and 

 nephridia. 



destroyed, apparently without injury to other cells. The individuals thus 

 treated are unable to regenerate. 



Cellular Structure and Functions. The cellular structure of the earth- 

 worm includes tissues of all the principal classes — epithelial, sustentative, vascu- 

 lar, contractile, and nervous — although within these classes there is less extreme 

 specialization than in the corresponding tissues of vertebrates. As shown by 

 a transverse section of the body in the region of the stomach intestine (Fig. 

 14.14), the body wall is covered externally by a delicate, non-cellular cuticle, 

 perforated where gland cells and sensory cells are exposed at their outer ends. 

 The cuticle is secreted by the underlying epidermis, a single layer consisting 

 of columnar epithelial cells. Scattered among the epithelial cells are numerous 

 receptor cells and the gland cells that secrete the fluid found upon the surface 

 of the body. Beneath the thin basement membrane upon which the epidermis 

 rests is a circular muscle layer composed of non-striated fibers lying in a plane 

 parallel to that of the transverse section. Contraction of these muscles 

 decreases the diameter of the body, compresses the coelomic fluid, and results 

 in an increase in the length of the body. Within this outer muscular layer 

 there is a considerable amount of fibrous connective tissue; vascular tissue is 

 also abundant here, for this layer of the body wall is the site of gas exchange 

 between the blood and the external environment. Beneath the circular 



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