ON POLYPS. 



lodgment of the hydrseform mouths ; being sometimes composed of 

 hard and dense calcareous substance, or else flexible and horny in 

 its texture : such are the Corallida or family of corals, properly so 

 called. The internal central axis is, moreover, in another family, 

 composed of several pieces united together by the living crust 

 which secretes them ; and such individuals, being free and unat- 

 tached, are probably able to change their position at pleasure : 

 these form the family of Pennatula. These groups are, however, 

 merely modifications of the same general type of structure, although 

 differing in certain minor points of their organization, so as to render 

 an examination of each form needful for our purpose. 



(36.) Alcyonidce. This family includes several genera, known by 

 the names of Alcyonium, Lobularia, Cydonium, &c., being charac- 

 terized by having no solid axis developed in the interior of the com- 

 mon body. The Cydonium Fig. 5. 

 Mulleri (Jig. 5, 1,) will give t 

 the reader a good idea of the 

 general appearance of one of 

 these compound animals. The 

 central mass, or polypary, is 

 entirely soft, being of a gelati- 

 nous or rather subcartilagi- 

 nous texture. Its density varies 

 with the state of the animal, 

 being more firm when the crea- 

 ture is contracted or hardened 

 by immersion in spirits of 

 wine, than when alive and ex- 

 panded. Upon cutting into 

 it, it is found to be intersected 

 by tough fibrous bands, and 

 not unfrequently contains calcareous spicula dispersed through its 

 substance ; no muscular fibre or nervous matter has ever been de- 

 tected in its composition, and its interior is permeated by nume- 

 rous wide canals variously disposed. The alcyonidse, therefore, may 

 justly be looked upon as intimately related to the sponges in the 

 structure of their common body, differing from them principally in 

 the polyps which occupy the cells upon their surface. 



(37.) The polyps which fill these cells resemble so many hydra in 

 their external configuration, from which, however, they differ in the 

 number of tentacula surrounding the mouth. In the hydra we 



