Pilidium of Certain N'emerteans. 253 



In C marginatus (Fig. 5, PI. xxxv), these organs develop much 

 moij^ rapidly and to a much greater extent. Their size is still 

 greater in C. lactexis, as may be seen from Wilson's figure (32). 



Their epithelial cells are thicker and correspondingly more numer- 

 ous than over the rest of the body. The free edge of each lappet is 

 thickened and covered with especially strong cilia. 



The muscular system is more highly developed here than in any 

 other part of the body excepting that immediately surrounding the 

 apical plate. In life the shape of the lobes is continually changing, 

 and violent contractions are not infrequently observed. The dis- 

 tribution of the muscular fibers will be described below. In the 

 optical sections shown in Figs. 2 and 4, PI. xxxiv, the relations of 

 the muscular cells and fibers to the epithelium are indicated. 



Besides the cells and fibers of the muscular system the cavity of 

 the lappets contains a considerable number of mesenchyme cells 

 freely suspended in the slightly gelatinous fluid which fills the whole 

 cavity of the body. These mesenchyme cells are irregular in form, 

 sometimes showing amoeboid movements, and are often conspicuous 

 from their yellowish or brownish color. 



The Nervous System. 



Salensky (28) has described for the pilidium a highly developed 

 and complex system of nerve-cells and nerve-fibers. The nervous 

 structures develop, as does the muscular system, out of the amoeboid 

 cells of the larval mesenchyme. Both bipolar and multipolar cells 

 are said to be present, and are provided with numerous, branching 

 processes. 



Salensky considers that the cells of the apical plate are highly 

 sensory, and finds that manj^ of them are continued internally into 

 very fine nerve-fibers. These run in company with the muscular 

 fibers to the anterior border of the esophagus and to the lappets. 

 He likewise finds a band of nerve-cells and nerve-fibers, constituting 

 what he calls a nerve-ring, reaching completely around the margins 

 of the pilidium and extending to the borders of the lappets. At the 

 anterior, upper border of each lappet he finds a ganglionated swell- 

 ing in the nerve-ring. This he considers the central organ of the 

 nervous system. Finally, he considers that some of the cells lining 

 the intestine belong to the category of nerve-cells. 



These last- mentioned are obviously nothing but glandular cells 

 which are partially filled with a deeply-staining secretion, as de- 



