Natural History of British Zoophytes. 



237 



same type, have made considerable advances towards complexity of 

 organization, and their relation to the polypidom is entirely alter- 

 ed. Hitherto the polypidom has been, what its name imports, a 

 cell for retreat in danger, and in ordinary an extravasular insensi- 

 ble sheath to protect the contained animal from the rude contact of 

 the circumfluent element ; but now we find it occupying an inter- 

 nal position, and instead of a covering it has become a sort of prop 

 or skeleton to a fleshy crust in which the polypes are immersed. 

 In the Alcyonium this interior support is scarcely to be recognized 

 in some calcareous spicula scattered through the central mass, but 

 in Pennatula it forms a bone stretched like a vertebral column from 

 one extremity to the other, and in Gorgonia it is ramified into 

 branches after the manner of a tree. It is this axis, under what- 

 ever shape it appears, which is the true analogue of the polypidom 

 of the ascidian and hydraform polypes, although the name certainly 

 has no suitableness here, for the polypes cannot only not nestle in 

 that which is uncellular, but they have no immediate connection 

 with it. They, as already mentioned, are found lodged in a sort of 

 cell (Fig. 4. a.) exca- Fig. 4. 



vated in an exterior sar- 

 coid crust, which con- 

 stitutes the main bulk 

 of the polypiferous 

 mass., and which, in 

 fact, is nothing more 

 than a production of 

 the bases and outer 

 skin of the polypes 

 hardened by a deposi- 

 tion of calcareous gra- 

 nules and spicula, and 

 made more coriaceous 

 in texture, to bear with 

 impunity the contact 

 and ruffling of the wa- 

 ter. * This crust is 

 accordingly a living- 

 irritable structure, permeated by tubes prolonged from the po- 



* " Lorsqu'on observe les Alcyons dans leur etat naturel, la ligne de demarca- 

 tion entre ces deux parties parait bien tranchee, et on pouvrait au premier abord, 

 croiie ces petits animaux loges dans des cellules au pourtour de l'ouverture des- 

 quelles ils adhereraient ; mais quand on eleve a l'aide d'un acide etendu d'eau, 



