156 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



Fig. 89. — a to /^. As in Fig. 88. i. Cavity of intestine, j. Epithelium 

 of intestine, k. Layer of circular muscular fibres around intestine. 

 I. Layer of longitudinal muscular fibres around intestine, m. Green layer 

 on outer surface of intestine, n. Dorsal vessel, o. "Liver." 



f. Notice that the muscular fibres of this layer do not 

 form a thin stratum on the inner surface of the layer of 

 circular fibres, but are arranged in bundles or leaflets, 

 which project into the body cavity so as to form a series 

 of parallel ridges. Each ridge consists of a central plate, 

 with muscular fibres on each side of it ; and, in transverse 

 section, has somewhat the appearance of a feather. A 

 longitudinal section of the body- wall will show that the 

 circular muscles have a similar feather-like structure when 

 cut across. 



ff. The body cavity is lined by a vascular layer (Fig. 

 89, c) which covers the inner surface of the muscular bun- 

 dles, and is rich in small vessels. 



7i. Covering these vessels and separating them from the 

 body cavity, the nuclei of a delicate layer of epithelial cells 

 may be made out in favorable specimens, with a high 

 power. 



II. The dissepiments between the somites. In a section 

 which contains the whole or a part of one of these parti- 

 tions, notice the muscular filires, which consist of: — 



a. A layer of circular fibres (Fig. 88, j) around the 

 digestive tract. 



b. A second set of circular fibres (Fig. 88, Jc) around 

 the nervous system and ventral blood-vessel. 



c. Fibres which radiate inwards from the body wall 

 towards the centre. 



d. A few nearly vertical fibres which run from the dor- 

 sal to the ventral surface. 



e. The surface of the partition is covered by an epithe- 

 lium, which is rather difficult to detect. 



