MUSEUM OF COMPARATIVE ZOOLOGY. 159 



e. Muscular Layers. 



Of the muscular layers the diagoual is not present in the introvert. 

 The circular layer, which is banded throughout the rest of the body, fuses 

 at the end of the integumentary areas into one continuous sheet, and 

 grows gradually less important anteriad, being almost entirely wanting 

 in the anterior zones. The longitudinal muscular bands do not fuse 

 until the middle of the posterior papillate zone is reached. From this 

 point anteriad they also become reduced so that in the smooth zone the 

 entire muscular layer measures but 70 to 90 /x in thickness. This rem- 

 nant passes over into the retractors in a manner to be de^-cribed in 

 treating of the tentacular musculature. 



2. Tentacular Fold. 



A cross section of the tentacular fold shows that it consists of two 

 layers of skin, which form the oral and aboral Avails of an irregular cav- 

 ity, traversed perpendicularly by numerous trabeculse binding the two 

 sides together (Fig. 3). This cavity is the extension of the so-called 

 blood system, and is often found more or less filled with a coagulum. 

 The character of the oral and aboral walls of this cavity differs : the 

 structure of the oral portion will be considered first. 



a. Oral Wall. 



The cuticula (Plate III. Fig. 23) is extremely thin, never exceeding 

 2 fi, and usually appearing as a fine double contour. It is pierced by 

 many pores for the exit of the fine cilia, which cover this surface from 

 the apex of the fold down into the mouth. Evidently the inversion of 

 surfaces in the retracted condition of the introvert led Selenka ('83, 

 p. xvii) and others to regard the oral surface of the tentacles as with- 

 out cilia, and to maintain that the aboral surface was ciliated, exactly 

 the reverse of which is true. 



The hypodermis (Plate III. Fig. 23) is composed of very high cells, 

 which are in contact merely by their distal ends. Proximally they are 

 prolonged into delicate processes, by which they are attached to the base- 

 ment membrane. These cells are of the type of filamentous cells (Haut- 

 fadenzellen) described by Eisig ('87, p. 300). Lying nearly in the centre 

 of the cell is the elliptical granular nucleus, which measures 11 by 4 yu.. 

 These cells are exactly similar to those contained in the sensory organs 

 before described. Some such cells are seen in Figure 21, r/, /(Plate II.). 

 In addition to these there are occasional cells in the hypoderm, the nuclei 



