ANTHOZOA HYDROIDA. 7 



ties, we find the denticles or cuplike cells of the polypes 

 arranged in a determinate order, either sessile or elevated 

 on a stalk. (Fig. 2, a.) Though of the same substance, 

 the cell is something more than a simple expansion 

 of the stem or branch, for near its base there is a dis- 

 tinct partition or diaphragm on which the body of the polype 

 rests, with a plain or tubulous perforation in the centre, 

 through which the connection between the individual polype 

 and the common medullary pulp is retained. Besides the 

 cells there are found, at certain seasons, a larger sort of vesicles 

 (fig. 2, J), readily distinguished from the others by their size 

 and the irregularity of their distribution, and destined to con- 

 tain and maturate the ovules. 



The polypidoms, when dried, are for the most part of a yel- 

 lowish or horn colour. " When they are immersed in water, 

 they recover the same form they appeared in when fresh in the 

 sea ; and soon become filled with the liquid. This gives them 

 a semitransparent amber colour, and makes them very elas- 

 tic." * Their material appears to be analogous to horn or 

 condensed albumen, which is moulded into a homogeneous 

 investing sheath, for the protection of the semifluid pulpous 

 body. It seems to be in fact a sort of hardened epidermis, 

 at first in contact and partial adhesion with the living in- 

 terior pulp, from which it is subsequently detached, in the 

 natural progress of its consolidation, by a process of shrivel- 

 ling in the soft matter, and by the motions and efforts of the 

 polypes themselves.-f- Link says that the experiments he has 

 made on the Plumularia falcata and the Sertularia cupressina 

 have led him to adopt the opinion of Cavolini and Schweigger, 

 that this sheath is vascular and organized, for, under a very 

 powerful magnifier, he has seen coloured vessels ramified in 

 the stem and branches of these polypidoms. He is also 

 certain that their stems are often increased with age by con- 

 contrary, grow only on these exposures, and never on the south. Sometimes their 

 position is varied according to latitude, and the shores inclined towards the south, in 

 temperate or cold countries, produce the same species as the northern exposures in 

 equatorial regions : in general, their branches appear directed towards the main sea." 

 Corall. Flex. Introd. p. L. 



* Ellis, English Corallines, p. 3. 



t See Lister's Observations in Phil. Trans. 1834, p. 374 ; and Lam. Anim. a. 

 'Vert. ii. p. 119. 2de 6dit. 



