THE CCELOMATA INVERTEBRATA 



189 



of a thin transparent membrane, composed of two sets of inter- 

 crossing fibres perforated by fine pores. 2. The ectoderm or epi- 

 dermis below the cuticle is composed of short columnar cells, forming 

 a simple epithelium, and containing a large number of mucous 

 secreting gland cells, and also the sensory cells arranged singly or in 

 groups. In the region of the clitellum the epidermal cells are greatly 

 enlarged and mostly glandular. The blood capillaries actually 

 penetrate this modified part of the epithelium a very unusual 



m 



m 



FIG. 60. Transverse section of the earthworm behind the clitellum. 



a.c., cavity of the digestive tract ; c., cuticle ; cos., ccelom ; c.m., circular muscles ; c.v., 

 circular vessel ; d.v., dorsal vessel ; hy., hypodermis ; l.nt., longitudinal muscles ; n.c., ventral 

 nerve chain ; p.e., peritoneal endothelium ; s., seta ; s.g., setigerous gland ; s.t'.v., sub-intestinal 

 vessel ; s.m., muscle connecting two groups of setae on the same side ; ty., typhlososle. From 

 Sedgwick and Wilson. 



occurrence in the animal kingdom. Certain insinkings of the 

 epidermis form the setigerous sacs, in which the setae are lodged. 

 Each is the product of one cell, and as the setae are constantly being 

 worn out and replaced there are always setae to be found within the 

 sac in various stages of development. They lie in a group of active 

 glandular cells which surround the bases of the two sacs in each pair, 

 and these masses constitute the setigerous glands. 3. Next to the 

 epidermis we find a moderately thin layer of circular muscle fibres. 



