THE PHYLUM ANNELIDA 



muscle laver is a laver of longitudinal muscle fibers; these fibers extend in 

 planes perpendicular to that of the transverse section. In such a section the 

 inner muscular layer has a feathery appearance, because its fibers are ar- 

 ranged along the surfaces of sheets of connective tissue, to which they are 

 attached. Contraction of the longitudinal muscles is antagonistic to the 

 action of the circular muscles; they shorten the body, compressing the coe- 

 lomic fluid and resulting in an increase in the diameter of the worm. Co- 

 ordinated, localized contraction and relaxation of these two muscle layers are 

 responsible for the peristaltic locomotor movements of the worm, operating in 

 conjunction with changes in the positions of the setae. The layer of longitu- 

 dinal muscle fibers is bounded internally by the parietal peritoneum, the lining 

 of the coelomic cavities, consisting of a squamous epithelium of mesodermal 

 origin. 



The internal surface of the body wall, the anterior and posterior faces of 

 the septa, and all other parts structurally related to the coelom are covered 

 by the continuous peritoneal epithelium. The layer covering the outside of 

 the digestive tube is spoken of as the visceral peritoneum, as in vertebrates. 

 In the earthworm and related annelids, this layer is modified from the usual 

 squamous type to form a columnar epithelium, composed of the chloragogue 

 cells referred to on page 409 in connection with excretion. Beneath this 

 layer the gut wall consists of the following components: a thin submucosa, 

 made up of connective tissue; a layer of non-striated muscle fibers, both 

 circular and longitudinal in orientation; and a single-layered epithelium 



Fig. 14.15. Asexual reproduc- 

 tion in aquatic oligochaetes. A, 

 Chaetos,aster diaphanus; B, Tubi- 

 fex tubifex. (Photographs by 

 Bassett Maguire, Jr.) 



