ON POLYPS. 37 



extent, surrounds the tubular cell. Generally many polyps of the 

 same polypary form these partitions at the same time and upon the 

 same plane. In this case the gelatinous margins of the folded mem- 

 brane unite, no space is left ; and they ultimately become most inti- 

 mately soldered together, and the solid plane or stage (fig. 8) is 

 formed. If the animal constructs its partition against a tube already 

 perfect and solidified, it fixes its collar to its sides, so that the point 

 of junction is imperceptible ; but when it is quite insulated, as at &, 

 Jig. 8, the horizontal collar is still formed, and it then assumes 

 somewhat of an octagonal shape. The tube-forming membrane 

 exhibits no appearance of vessels or other traces of organization. 



When the polyp is withdrawn within its cell, its tentacles form 

 a cylindrical fasciculus {Jig- 9, c) ; the papillae which partially cover 

 them being laid upon each other like the leaflets of some mimosa 

 when asleep. 



The protrusion of the creature from its tube is accomplished by 

 the contraction of the membrane, 6, inserted into its neck. 



(52.) How the eggs formed upon the oviferous filaments issue from 

 the polyp, has not been ascertained : it is most probable, from their 

 size, that they are not expelled during the life of the parent ; but 

 that, when it dies, the eggs all come out of the tube, except one, 

 which developes itself in the old cell ; the rest fixing themselves 

 upon the neighbouring stage, there to form a new story of tubes. 

 The germs, during the first period of their developement, have no 

 organs distinguishable, not even the rudiment of a tube ; each ap- 

 pears to consist of a simple gelatinous membrane folded upon itself, 

 (Jig. 9 ; 4, c,) and forming upon the stage upon which it is fixed a 

 little tubercle resembling a small Zoanthus or other naked zoophyte. 

 This tubercle gradually elongates, and assumes the form of a polyp, 

 provided with all its organs ; but the sac which encloses it is still 

 gelatinous at its upper part, and membranous near the base, 

 (Jig. 9 ; 4, 6,) where it gradually diminishes in thickness, and, 

 becoming calcareous, gives to the animal the general appearance of 

 its original. 



(53.) In Tubularia indivisa the structure of the tentacula around 

 the mouth is different from what has been described in Tubipora mu- 

 sica, although in the principal points of its structure the resemblance 

 between the two is very great ; when the Tubularia is expanded, its 

 protruded portion is seen to be furnished with two circles of arms, 

 one placed around the opening of the mouth, the other at a consi- 

 derable distance beneath it, (Jig. 10 ; 1,) and nearly on a level with 

 the inferior circle a second aperture (Jig> 10 ; 1, a) is observable, 



