W. H. Coe — Anatomy of Cerehratuliis lacteus. 485 



few characteristic nuclei are imbedded and through which fibres pass 

 in various directions between the nerves and muscles in the body- 

 wall and the organs which the tissue surrounds. Wherever this 

 tissue comes in contact with any other tissue, and especially sur- 

 rounding the blood-vessels, we find a mantle of large cells. These 

 parenchyma-cells (Plate XIII, figs. 13, 14, pc) possess large, oval 

 nuclei which are imbedded in small masses of granular protoplasm 

 which in the preparations have shrunken away from the surrounding 

 walls. 



JBody- cavity. — In Nemerteans we find no space corresponding to 

 the body-cavity of the Annelids, the space about the alimentary 

 canal being filled up with the gelatinous tissue, or parenchyma, 

 described above. The cavity of the proboscis-sheath, or rhyncho- 

 coelom, is often looked upon as homologous with the body-cavity. 

 It is not uncommon to find a small split (Plate XIV, fig. 10,/) 

 between the outer border of the intestinal caeca and the surrounding 

 gelatinous tissue. In some species of Nemerteans* this split is stated 

 to be lined with a special membrane, although in this species it could 

 not be found, the split appearing merely as an artificial shrinking 

 of the intestinal wall away from the surrounding gelatinous tissue. 



The Prohoscis-sheath. 



The proboscis opens to the exterior through a terminally placed 

 pore (Plate XII, fig. l^po) which leads into a cylindrical chamber 

 at the posterior end of which the anterior end of the proboscis is 

 attached to the cephalic musculature. This chamber has been called 

 by Hubrecht (7) rhynchodmum (Plate X, fig. 1 ; Plate XI, fig. 2 ; 

 Plate XII, figs. 1, 2, r). It continues backward into the proboscis- 

 cavity but is sharply marked off from the latter by a strong sphinc- 

 ter. The glandular and ciliated epithelium of the body passes 

 gradually into the simple, columnar epithelium of the rhynchodaeum 

 and this, in turn, into the more or less stratified epithelium of the 

 proboscis. Although the cell-walls of the columnar epithelium 

 (Plate XII, fig. 8) of the rhynchodseum are not very distinct, the oval 

 nuclei are regularly arranged with their long axes at right angles to 

 the lumen. Each cell bears a cluster of moderately long cilia. In 

 the anterior portion of the rhynchodseum a special musculature is 

 wanting and the epithelium is separated from the cephalic muscles 

 only by a thin layer of connective tissue, the basement membrane. 

 Towards the posterior half of the chamber a layer of circular mus- 



* Cf. Salensky (15). 



