ON THE LTFE-HISTORT OF PBDIOELLINA. 153 



epithelial cells and some of their nuclei pass bodily into the 

 lumen of the organ (cf. figs. 8 and 9), where they are found 

 quite free at later stages. The more projecting parts of the 

 epistome and of the anal cone lose most of their component 

 cells. The cilia of the former become indistinct, the cell-sub- 

 stance itself obviously degenerating (fig. 9). Ultimately ciliated 

 portions of the cells are thrown off into the vestibule (figs. 9 

 and 12), in which they can be discovered until a very late stage 

 in the metamorphosis. They no doubt leave the vestibule 

 either by the mouth or by the (adult) vestibular aperture, 

 ■when the latter is formed. 



The histolysis of the ventral portion of the vestibule (fig. 9, 

 V. V.) similarly results in the passage of fragments of cells into 

 its own cavity. 



This process is again illustrated by fig. 12, a section passing 

 in the plane of the line e f in fig. 9. The permanent vestibule 

 is in this section (cf. fig. 16) completely separated from the 

 degenerating portion, its lumen, like that of the latter, con- 

 taining fragments of degenerating cells. 



The ventral division of the vestibule (v. v.) in fig. 9 occupies 

 the position of the future stalk, and in later stages its cavity 

 becomes more and more reduced until it finally atrophies. 

 During this process, the cells previously found in its lumen 

 disappear. In sections parallel to the plane of attachment the 

 cavity (just before its atrophy) appears as a fine tube surrounded 

 by a series of elongated cells radiating from it towards the 

 body wall. It is very tempting to assume that these cells are 

 phagocytes, engaged in the destruction of the vestibule. After 

 the atrophy of the latter, its place is occupied by numerous 

 " globules" (fig. 10), which will themselves be replaced by 

 ordinary connective-tissue corpuscles (fig. 13) . 



The same assertion may be made of other parts of the 

 " primary body cavity," which is at the stage of fig. 9 almost 

 completely filled with " globules/' resulting from the histolysis 

 of the brain, the sucker, the tissue at the base of the epistome 

 and anal cone, and other larval structures. When the primary 

 individual is mature the " globules" have disappeared, and are 



12 



