MUSEUM OF COMPARATIVE ZOOLOGY. 31 



of the body wall. It is iuteresting to find them persisting in the fu- 

 niculas of Paludicella, beneath the mesodermal covering, although there 

 is apparently no muscularis developed in the body wall of this region. 



8. The Formation of the Neck and Atrial Opening. 



This is the last act in the history of the polypide that I shall con- 

 sider. The body wall around the neck of the polypide continues to 

 possess a less differentiated character than the remaining portion for 

 some time after the oral tentacles have undergone their revolution. 

 One still sees the cells of this region dividing, and the body wall is 

 gradually protruded at this point above the general level. (Plate II. 

 Fig. 14, cev. pyJ.) The neck of the polypide to which the kamptoderm 

 is attached consists, at a somewhat earlier stage than that just referred 

 to, of a disk of greatly elongated columnar cells in the centre of which 

 there is a distinct notch caused by the presence of shorter cells at that 

 point. (Plate VI. Fig. 63 b.) At the inner ends of the columnar cells 

 of the neck lies a flat epithelium quite sharply marked off from the 

 latter, but which is nevertheless undoubtedly derived from the same 

 source as the columnar cells and the inner layer of the bud. This flat 

 layer is directly continuous with the inner layer of the kamptoderm. 

 At a later stage, the columnar cells of the ectoderm become elongated 

 still more, and lose their staining capabilities at their outer ends. Still 

 later one sees them arranged in the form of a cup whose cavity is sep- 

 arated from the outside world only by a cuticula which becomes slightly 

 invaginated at this point. The cells are soon found with their long 

 axes perpendicular to the edge of the cavity they line. 



There is one point that I have not been able to determine ; namely, 

 how the new cuticula, which is certainly formed at the ends of the cells 

 which lie next to the cavity, becomes continuous with the old cuticula 

 of the non-invaginated body wall, as it is in Figure 50 (Plate Y.). The 

 presence on the new unstainable cuticula of the remains of the staiuable 

 one, whose origin I have already discussed at length, may serve as a 

 guide to the limits of the old cuticula. The new cuticula is being secreted 

 by cells lying deep in the inner end of the neck, and apparently in one 

 rod-like mass. Unfortunately, I lack stages between this figure and 

 Figure 45 (Plate V.), which shows the neck of a nearly or quite adult 

 polypide cut lengthwise. The solid cuticular rod has now become a hol- 

 low cylinder, whose inner (deep) edge is embedded in the deep-lying cells 

 of the neck. Moreover, one finds superficial to the cuticula of the gen- 

 eral body wall a second cuticular cylinder, which is free at its outer end, 



