460 poLVPi, 



hollow ovoid, open at both ends and with two envelopes, both per- 

 forated by meshes like the Retepora*. 



In the fourth tribe the animal rind or bark encloses a mere fleshy 

 substance without an axis either osseous or horny. In 



Alcyomum, Lin., 



As in the Pennatulae, we observe Polypi with eight denticulated 

 arms, and intestines prolonged into the common mass of the ovaries: 

 but this mass is not supported by an osseous axis ; it is always fixed 

 to the body; and where it is drawn out into trunks and branches, 

 nothing is found internally, but a gelatinous substance traversed 

 by numerous canals surrounded Avitli fibrous membranes. The bark 

 is harder and excavated by cells, into which the Polypi withdraw 

 more or less entirely. The 



A. digitatum, Ell., Corall., XXXII, which is divided into thick 



and short branches; and the A. exos, where branches are more 



slender, of a beautiful red, &c., are very abundant in European 



seas. 



Linnseus and his successors have rather lightly united to the Al- 



cyonia various marine bodies of different tissues but always Avithout 



any visible Polypi. Such are 



Thetiiya, Lam., 

 Where we observe the interior roughened with long, siliceous, spiral 

 lines, which unite on a similarly siliceous and central nucleus. The 

 crust, as in Spongia, presents two sorts of holes ; the first, closed by 

 a sort of grating, must be for the intermission of water, and the second, 

 which are gaping, for its exitf. 



After the Alcyonia are also placed the 



Spongia, Lin. +, 



Or Sponges ; marine, fibrous bodies whose only sensible portion ap- 

 pears to be a sort of tenuous gelatine, which dries off, scarcely leaving 

 a trace of it, and in which neither Polypi nor other moving parts 

 have yet been discovered. Living Sponges are said to exhibit a sort 

 of tremulousness or contraction when they are touched ; it is also 

 affirmed that the pores, with their superficies, are perforated, and 



* The Releporile, Bo'sc, Journ. de Phys., June 1826. For these genera of little 

 free Millepora, see also the work of Lamoiiroux just quoted. 



t See Messrs. Audouin and Milne Edwards, Ann. des So. Nat., XV, p. 17. 



N.B. A great portion of the Alci/onia of Lam. belong in reality to his Thethy». 



Add the fossil genera, which M. Lainouroux thinks be can approximate to the 

 Alcyonia or Thethyx: his Halliroe, and those which form his order of the Ac- 

 tiniaria; his Chenondopora, Hippalin.e, Limnore^, Sere.e, &c. — all pro- 

 ductions of which the nature is more or less problematical. 



X The genus of the Sponges is extremely rich in curious species, and would well 

 repay its study. M. de Lamarck — An. sans Vert., II, 345, et seq. — will prove an 

 excellent guide. See also the important Memoir of M. Grant, Ann. des Sc. Nat., 

 XI,pl. xvi. 



