W. H. Coe — Anatomy of Cerehratulus lacteus. 495 



the lateral vessels with the dorsal vessel at the posterior end of the 

 body. 



Histology. — The walls of the lacunae, and of those vessels which 

 are surrounded by the body-muscles, have a different structure than 

 those of the vessels which are surrounded by the parenchyma. In 

 general, the former are destitute of a muscular coat, while the latter 

 (Plate XIII, figs. 13, 14) have more or less strong walls of circular 

 muscles. All these vessels have a delicate endothelial lining (en') in 

 which the small, oval nuclei are quite conspicuous on account of 

 their affinity for staining fluids. The cell-outlines, however, cannot 

 well be made out. In most cases the endothelium is thrown up into 

 minute folds. Outside this endothelial lining are a few delicate 

 longitudinal muscular fibres, and a layer of circular muscles (m) 

 made up of very fine fibres with exceedingly few nuclei. The thick- 

 ness of this layer varies much in different vessels, and, as a rule, is 

 greater in those vessels of the intestinal than in those of the oesopha- 

 gal region. Outside the muscular layer is a coating of large, 

 wedge-shaped parenchyma-cells {pc) having the thin ends attached 

 close to the muscular wall, while the outer thickened ends with their 

 rounded edges are surrounded by gelatinous tissue. These cells are 

 not unlike the cells found throughout the gelatinous tissue, especially 

 where it comes in contact with other tissues. They are also found 

 about the nephridial vessels, though to a much smaller extent. Each 

 cell is provided with an oval nucleus. Between the parenchyma- 

 cells are fine fibres which connect the blood-vessels with the other 

 tissues. In the lacunae of the head the endothelium is separated 

 from the cephalic musculature only by a thin sheet of connective 

 tissue. No muscles or parenchyma-cells are found. 



The dorsal vessel (Plate XIII, fig. 9) is provided with a much 

 thicker muscular wall than is found in any of the other vessels. 

 Outside of the muscular wall, and beneath the much-folded endothe- 

 lium of the rhynchocoelom, is a thick layer apparently made up of a 

 mass of elongated parenchyma-cells (pc) piled one on top of another, 

 and which have a dense protoplasm and oval nuclei. Between the 

 cells run bundles of fibrous tissue from the muscular layer of the 

 vessel towards the endothelium of the proboscis-sheath {en). Soon 

 after the origin of the dorsal vessel from the ventral commissure 

 of the lateral lacunae, and while it is still surrounded by the muscular 

 fibres of the proboscis-sheath, these parenchyma-cells appear around 

 it. Likewise when the vessel leaves the proboscis-sheath in the pos- 

 terior oesophagal region a thick coating of these cells continues, and 



