484 W. H. Coe — Anatomy of Cerehratulus lacteus. 



fig. 10) of various sizes, each containing a small number of separate 

 fibres and surrounded by a sheath of connective tissue (c) which 

 contains large, oval nuclei. In some cases the sheath is very thin 

 and delicate, while in others it equals in thickness the diameter of 

 the muscle-bundle. Delicate connective tissue fibres from this sheath 

 surround the individual muscle-fibres i^mf). Between the muscle- 

 bundles run also numerous nerve-fibres. 



In the head (Plate XI, fig. 2) the muscles are not separated into 

 distinct layers as they are in the rest of the body, but the cephalic 

 musculature is made up of an irregular network of fibres, muscular 

 and nervous, running in all directions. 



In the region of the mouth, the muscular layer is, of necessity, 

 incomplete and ends on each side of the mouth-opening. A thick band 

 of horizontal muscles (Plate X, fig. 4, Am), lying on a level with the 

 lateral cords between the mouth and proboscis-sheath, connects the 

 right and left portions of the circular layer (cm), so that in reality 

 there is a provisional circular musculature which is complete only 

 about the proboscis-sheath and lateral lacunae. 



The cell structure of the muscular tissues is difficult to make out 

 in most parts of the body because of the excessive fineness of the 

 fibres and the small number of nuclei which can be distinguished. 

 In the large fibres in the anterior portion of the proboscis-sheath, 

 however, each fibre (Plate XV, fig. 12) appears in cross section to 

 have a circular or angular outline, inside of which is the unstained 

 contractile substance and near one side an oval or flattened, often 

 crescent-shaped, nucleus (n). Each muscle-fibre, therefore, may be 

 conceived to be a single, greatly elongated cell with a single flat- 

 tened nucleus just beneath the cell-wall. In Carinella the muscular 

 fibres (Plate XV, fig. 11) are considerably larger and the nuclei 

 show very distinctly. 



The Gelatinous Tissue and Body-cavity. 



Internal to the body-wall and surrounding the proboscis-sheath, 

 alimentary canal, blood and nephridial vessels, etc., is a mass of 

 jelly-like connective tissue called by Burger (2) the " parenchyma " 

 and by Hubrecht (7) ''gelatinous tissue." This gelatinous tissue 

 seems to be united directly with the connective tissue surrounding 

 the muscle-bundles in the body-wall. It is present throughout the 

 length of the body and occurs most abundantly as broad, thick 

 bands above and below the alimentary canal (Plate XI, fig. 3 ; Plate 

 XIV, fig. 8, gd). It is a structureless, transparent tissue in which a 



