1901.] NATURAL SCIENCES OF PHILADELPHIA. 



cells. The nuclei of the component connective tissue cells are seen 

 with difficulty, but occur here and there embedded in the fibres. 

 With a low power the basement layer has a homogeneous, rather 

 gelatinous appearance, but with higher magnification its true fibrous 

 structure is seen. The avei'age height of the basement layer in the 

 head region is about .006 mm., but it is not a layer of uniform 

 thickness, for its outer surface is thrown into a series of small eleva- 

 tions and depressions. The ridges on the surface bear the stalks 

 of the supporting cells, while the gland cells are inserted into the 

 pits or _ depressions. The basement layer does not stain with 

 hsematoxylin-eosin, but takes a faint pink with the Biondi-Ehrlich 

 stain. 



An epithelial musculature of circular muscle fibres, fig. 2, 

 Ei^.m., is present immediately beneath the basement layer. It is 

 especially well developed in the head region, consisting of a num- 

 ber of fibres like those of the body wall. Posterior to the oesopha- 

 geal region the epithelial muscle layer becomes very thin and finally 

 disappears, but reappears at the posterior end of the body. 



h. The Cutis. — The cutis is defined by Burger (1895) as the 

 subepithelial glandular layer, usually containing numerous nmscle 

 fibres, which is found in the Heteronemerteans. 



In Zygeupolia any distinction between the outer longitudinal 

 muscle layer and the cutis would be merely an artificial one. The 

 fibres of the outer longitudinal muscle layer extend from the cir- 

 cular layer out to the epithelium of the body wall, and although 

 subepithelial gland cells are present, frequently in great numbers, 

 they are not restricted to the peripheral portion of the layer, but 

 often extend in as far as the circular muscle. It is thus evident 

 that in Zygeupolia the term cutis is synonymous with outer longi- 

 tudinal muscle layer. In this respect Zygeupolia differs greatly 

 from the genus Eupolia, in which a cutis distinct from the outer 

 longitudinal muscle layer is present. 



The finer structure of the cutis, or of the outer longitudinal 

 muscle layer, may be seen in fig. 2. Each longitudinal muscle 

 fibre, L.M.j., is surrounded by a sheath of connective tissue, com- 

 posed of the slender processes of the connective tissue cells which 

 are present between the muscle fibres. These cells are greatly 

 branched, their nuclei, Cn.T.N., are oval, and contain a small 

 amount of chromatin. Slender radial muscle fibres, r.m.f., trav- 



