272 MOLLUSCA. 



these rays, and the opening terminates in a common cavity, which is in the 

 centre of the star. If an orifice be irritated, but a single animal contracts; if 

 the centre be touched they all contract These very small animals attach 

 themselves to certain Ascidise, Fuci, &c. 



In some particular species, three or four stars appeared to be piled one on 

 the other. 



PYROSOMA, Peron. 



The Pyrosomae unite in great numbers, forming a large hollow cylinder, open 

 at one end and closed at the other, which swims in the ocean by the alternate 

 contraction and dilatation of the individual animals which compose it. The 

 latter terminate in a point on the exterior, so that the whole external surface 

 of the tube is bristled with them ; the branchial orifices are pierced near these 

 points. A Pyrosoma may thus be compared to a great number of stars of Bo- 

 trylli strung together, the whole of which is moveable. 



The Mediterranean and the Ocean produce large species, the animals of 

 which are arranged with but little regularity. They exhibit a phosphorescent 

 appearance during the night 



A smaller species is also known where the animals are arranged in very 

 regular rings. 



The species of the remainder that are known are all fixed, and till now have 

 been confounded with the Alcyonia. The visceral bundle of each individual is 

 more or less extended into the common cartilaginous or gelatinous mass, more 

 or less narrowed or dilated in certain points; but each orifice always forms a 

 little six-rayed star on the surface. We unite them all under the name of 



POLYCLINUM. 



Some of them are extended over bodies like fleshy crests. Others project in a 

 conical or globular mass; or expand into a disk comparable to that of a flower 

 or of an Actinia; or are elongated into cylindrical branches supported by 

 slender pedicles, &c. ; or form parallel cylinders. 



CLASS V. 



BRACHIOPODA. 



THE Mollusca Brachiopoda, like the Acephala, have a bilobed mantle, 

 which is always open. Instead of feet they arc provided with two 

 fleshy arms, furnished with numerous filaments, which they can pro- 

 trude from and draw into the shell. The mouth is between the base 

 of the arms. 



All the Brachiopoda are invested with bivalve shells, fixed and 

 immoveable. But three genera are known. 



