Natural History of British Zoophytes. 321 



and almost always jointed at short and regular intervals, the joint be- 

 ing- a mere break in the continuity of the sheath without any cha- 

 racter of a proper hinge, and evidently formed by regular periodical 

 interruptions in the growth of the polypidoms. Along- their sides, or 

 at the extremities, we find the denticles or cup-like cells of the poly- 

 pes arranged in a determinate order, either sessile or elevated on a 

 stalk, (Fig-, 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 distinct 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, (Fig. 5.) * Besides the cells there are found, 

 at certain seasons, a larg-er sort of vesicles, readily distinguished from 

 the others by their size and the irregularity of their distribution. — 

 The more robust tribes grow erect, and, being flexible and elastic, 

 yield readily to the waves and currents ; but some of the very deli- 

 cate species avoid a shock for which they are unequal by creeping 

 along the surface. 



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

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

 the same form they appeared in when fresh in the sea ; and soon be- 

 come filled with the liquid. This gives them a semitransparent amber 

 colour, and makes them very elastic." f Their material appears to 

 be analogous to horn or condensed albumen, which is moulded into 

 a homogeneous investing sheath, in which no vessels or cells indicat- 

 ing a definite organization can be detected. J It seems to be in fact a 

 sort of hardened epidermis, at first in contact and partial adhesion 

 with the living interior pulp, from which it is subsequently detach- 

 ed, 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. ^ 



The polypes are placed in the cells within which, with the excep- 

 tion of the Tubularise, they can hide themselves entirely when danger 

 threatens. When at rest and in their native sites, they expand their 

 tentacula and push them far beyond the rim of their cups, in readi- 

 ness to arrest any small worm or crustaceous insect which may float 

 within their circle. These tentacula are always simple but rough- 

 ish, (Fig. c) andin the centre of the disk round which they arearrang- 



* Lister, in Phil. Trans. 1834, p. 371. f Ellis, English Corallines, p. 3. 



\ The contrary is maintained by Link, and it appears by Cavolini and 

 Schweigger, who assert that they have seen vessels I'amified in the stems and 

 branches of Sertulariadae Ann. des. Sc. Nat. Part. Bot. V. ii. p. 321. 



§ See Lister's Observations in Phil. Trans. 1834, p. 374. 



