INTRODUCTION. XVll 



The TENTACULAR SHEATH is ail important elemeut of 

 the structure, and a very characteristic feature of the 

 ordinary Polyzoau type (Woodcut, fig. ii. sh). It is a 

 membranous extension of the anterior part of the zooecium, 

 which, when the polypide is retracted, is inverted and 

 drawn in with it, closely surrounding the folded tentacles. 

 According to some writers * it is wholly composed of the 

 endocyst ; but Joliet refers it to the tissue which he has 

 named the endosarc, of which an account will be given 

 hereafter. The invagination of the sheath is due to its 

 attachment at its upper extremity round the base of the 

 crown of tentacles ; as the latter descends in obedience to 

 the summons of the retractor muscles it is, of course, 

 drawn down with it, and reversed as the finger of a glove 

 might be under similar circumstances, forming a protec- 

 tive case around it, which somewhat exceeds it in length. 

 By this arrangement the zooecium is completely closed ; 

 there is no real opening through which the polypide 

 passes. It is the upward movement of the tentacular 

 corona (to be explained hereafter) which carries with it and 

 everts the flexible sheath, and so permits the imprisoned 

 zooid a certain amount of communication with the outer 

 world ; but the cavity of the cell itself is sealed. The move- 

 ments of the polypide in the acts of expansion and retrac- 

 tion are limited to the eversion and inversion of the sheath ; 

 and only the corona is brought into immediate contact 



* AUmau and Nitselie. Ehlers, however {op. cit. p. 37), considers that 

 both ecto- and endocyst enter into its composition, though the former is 

 (in many species) less solid in this portion of the structure than in the rest 

 of the outer wall of the cell. Joliet holds that the tentacular sheath is a 

 derivative from the endosarc (see page xxxv), and not an invaginable portion 

 of the endocyst. He describes it as of the same nature as the funiculus, 

 i-esembling it in its mode of development, in its contractility, and in its 

 histological elements {op. cit. pp. 51, 52). I am inclined to think (hat this 

 may prove to be the true account of it. 



C 



