10 bulletin: museum of comparative zoology. 



are massed together in certain regions to form specialized sense 

 organs, wliicli are described below. 



Longitudinal and transverse muscular fibers occur in the midst of 

 the integument of the species of Carhstoma, while in the Lineidae 

 similar integumental muscles occur immediately beneath the epithelial 

 cells. 



Pigment. — The interstitial cells are often filled with pigment 

 granules, which commonly impart the peculiar coloring to the body, 

 and are often arranged in such a manner as to form distinctive spots, 

 lines, bands, or rings, characteristic of the different species. 



In other forms (as Tetrastemma bilineatum,) the pigment cells, 

 which determine the characteristic markings of the body, have 

 wandered beneath the integument, and have become situated in 

 localized areas in the midst of the muscular walls. 



In certain other species (as Cerebratulus marginatiis) the color 

 of the glandular secretions of the integument determines to a great 

 extent the natural color of the body, while in still other forms the 

 muscular layers are distinctly colored. 



Basement Layer and Cutis. 



Basement layer. — In all nemerteans belonging to the Paleo- 

 nemertea and Hoplonemertea, a layer of homogeneous hyaline con- 

 nective tissue is ^situated immediately beneath the integument, and in 

 most species of these groups forms a conspicuous basement layer 

 (PI. 13, figs. 81, 82). In some species the basement layer is fidly 

 equal to the integument in tliickness, wliile in other forms of the 

 same genus it may be but a small fraction of the thickness of the 

 integument. 



Numerous minute nuclei and a few delicate fibers are scattered in 

 this homogeneous basement layer, which is usually much more highly 

 developed anteriorly than towards the posterior end of the body. 

 This layer is thrown into folds and branching processes externally 

 to give a firm support to the integumental cells which rest upon it. 

 In the Heteronemertea, a fibrous network of connective tissue takes 

 the place of the homogeneous basement layer of the three other 

 orders. 



Cutis. — In most species of the Heteronemertea this network of 

 connective tissue fibers becomes of a very considerable thickness, 



