PHYLUM ANNELIDA 



103 



tine, and the intestine with its dorsal infolded typhlosole and outer covering of 

 peculiar large cells. In the body wall, the most conspicuous layer is the feathery 

 layer of longitudinal muscles, which is interrupted in eight places, four lateral 

 and four ventral for the insertion of the setae. These interruptions divide the 

 longitudinal muscle coat into bands, which were noted in the preceding paragraph 

 in the cut surface of the earthworm and should be identified again. Identify 

 the dorsal blood vessel lying above the typhlosole imbedded in the large pear- 

 shaped cells; the ventral blood vessel below the intestine to which it is attached 

 by a ventral mesentery; and the ventral nerve cord just below the ventral blood 

 vessel. In the coelome may be seen traces of nephridia, blood vessels, and septa. 

 Make a diagram of the section. 



b) Structure of the body wall: Examine with the high power, and study the 

 following layers: 



(1) Cuticle: The outermost layer, a thin non-cellular covering. 



(2) Epidermis: This contains about four kinds of cells. The most conspicu- 

 ous are the gland cells, large elliptical cells filled with granules, which are fore- 

 runners of mucus. These gland cells open to the surface by way of the pores 

 already noted in the cuticle. Between the gland cells are interstitial cells, 

 elongated cells with broadened ends. At the bases of the gland and interstitial 

 cells may be noted some small cells, which are sometimes considered to be a 

 second layer. The fourth kind of cell, sensory cells, cannot be seen without 

 special methods of staining (Hegner, Fig. 86, p. 167). 



(3) Circular muscle layer: This consists of very long slender muscle cells 

 imbedded in connective tissue, running circularly. 



(4) Longitudinal muscle layer : This consists of muscle cells like those of the 

 circular layer, but as they run longitudinally they appear in cross-sections as 

 circles or ellipses. These longitudinal muscle cells are mounted on plates or 

 septa of connective tissue, which extend out from the body wall at right angles 

 to it. This arrangement of the muscle cells along these septa gives a feathery 

 appearance in cross-section. The interruptions of the longitudinal muscles for 

 the setae have already been noted. 



(5) Peritoneum: The body wall is lined with peritoneum, a very thin layer 

 of cells applied to the inner surface of the longitudinal muscle layer. 



(6) Setae: If setae are present on your slide note their appearance. They 

 are slender, yellow, curved rods of the same composition as the cuticle and 

 like it secreted by the epidermis. The epidermis turns in to form a sheath 

 around each seta and a sort of cap at the inner end of the seta, where the 

 seta is secreted. From the epidermal sheath and cap muscles may be seen 

 extending to the circular muscle layer. These muscles serve to move the 

 setae in all directions. 



Draw a small portion of the body wall in great detail. 



