134 THE BIOLOGY OF HYDRA : 1961 



how the contents of the head could l^e released. It is hoped that 

 further study may clarify this intriguing situation. 



Another structural feature of the stenotele not previously de- 

 scribed is the occurrence on the base of each spine of a bulbous 

 enlargement (arrow). This arrangement would seem to insure a 

 firmer attachment of the spines to the invaginated capsular wall. 



Figure 2 also illustrates, in a transverse section through the 

 basal portion of a stenotele of a stage of maturity similar to that 

 of Figure 1, the bulbous enlargement (arrows) of the bases of the 

 spines. This figure is also interesting in that it includes a cross 

 section through the enlarged head of the tubule (HD). 



Figure 3, a transverse section through the apical portion of a 

 stenotele, just below the level of the operculum, reveals the inter- 

 locking relationship of the stylets, previously described by Chap- 

 man and Tilney (ref. 4 Fig. 12). In the present figure, however, the 

 stylets are closer together and no spines are included in the section. 



In an earlier paper (4), following the lead of previous workers 

 (e.g., ref. 7), it was stated that the capsular wall consists of 

 chitin or keratin. Since that paper was prepared, the author learned 

 of the work of Lenhoff ct al. (11), Johnson and Lenhoff (8) and 

 Lenlioff and Kline (10), in which it has been shown biochemical- 

 ly and histochemically that nematocyst capsules contain protein 

 which is probably a member of the collagen family of proteins. 

 Since one of the prominent fine structural features of collagen is 

 its characteristic periodicity, the electron micrographs of nemato- 

 cyst capsules were scrutinized carefully to determine whether any 

 indications of a periodic structure might be foimd. Figure 4, a trans- 

 verse section through the basal region of a stenotele, reveals the 

 presence, in the material of the invaginated capsular wall (ICW), 

 of a periodic structure with a 160 A periodicity. This is, of course, 

 a value one-fourth that of the usual 640 A period of collagen. It 

 should be noted that no fibrillar elements were observed in the 

 material of the capsule. Figure 5 shows a portion of the field of 

 Figure 4 at higher magnification. It is not immediately clear why 

 the invaginated capsular wall, which has been shown to be continu- 

 ous with the capsule proper (4), should reveal this periodic struc- 

 ture when the capsule proper does not. Nor is it clear why this 



