10 ANTHOZOA HYDROIDA. 



The polypes (fig. 2, c) are placed in the side and terminal 

 cells within which, with the exception of the Tubularinse, 

 they can hide themselves entirely when danger threatens. 

 The body is normally of a somewhat globular figure, and of 

 a nearly homogeneous composition, consisting of an aggre- 

 gation of vesicular granules held together by a semitrans- 

 parent glairy gelatine. It is very remarkably contractile at 

 every point, so that its form can be changed rapidly from a 

 globe to a cylinder, or distorted with swellings and constric- 

 tions ; and the tentacula, endowed equally with this contrac- 

 tile power, can be also shortened and extended at will, and 

 sometimes to an extent which is almost marvellous.* When 

 therefore the polypes have occasion to conceal themselves 

 within their cells, they are not necessitated to bend the body 

 in order to obtain sufficient space, but they shorten the body 

 and the tentacula at the same time by a process of condensa- 

 tion, causing the one to assume a more globular form, and the 

 other to dwindle down to mere knobs or papillae. 



The tentacula are irregular in number. They are always 

 simple and filiform, or rather tapered a little towards the ex- 

 tremity, and have their surface roughened more or less with 

 granules arranged in an imperfectly verticillate fashion. The 

 granules appear to be of a glandular nature ; but in the Hydra, 

 and perhaps in some others, they are also organs which con- 

 tain a singular apparatus to paralyse and kill the animalcules 

 the polype feeds upon. 



In the centre of the circle formed by the tentacula, on the 

 superior disk of the body, is placed the oral aperture, very 

 dilatable and sometimes capable of being elongated into a sort 

 of snout, but which is always unfurnished with any ciliary 

 or dental apparatus. It leads by a short passage into the 

 stomach, which is not a distinct sac, but a simple cavity ex- 



* This extensible capacity has been usually ascribed to an expansion and wider 

 separation of the vesicular granules of the organs, and their contractility to the con- 

 densation of the same granules ; but it would appear that the motions may be partly 

 ascribed to a muscular structure. Mr. Lister says, that, " in the substance of the 

 necks of the polypi (of Sertularia pumila), transverse lines were visible, bearing a 

 resemblance to those characteristic of voluntary muscles in the higher animals." 

 Phil. Trans. 1834, p. 371. Corda has described and delineated muscular bands in 

 the tentacula of the Hydra ; and M. Quatrefages, a regular muscular system in his 

 Synhydra. Ann. des Sc. Nat. xx. p. 238. 



