384 MOLLUSCA. 



ginous or gelatinous common mass, and more or less constricted and dilated at particular parts*; but each orifice 

 always represents on the surface a little star with six rays. We unite them all under the name of PobjcUnum. 

 Some cover foreio'u bodies like fleshy crusts ; others rise in conical or g;lobose masses. Others again expand into 

 a disk so as to have a distant resemblance to a flower or an Actinia ; or they are lengthened out into cylindrical 

 branches, supported by more slender pedicles ; or they are grouped into cylinders (Synoicum, Lam.). It even 

 appears from some recent observations that the Escharidae, hitherto arranged with polypiferous Zoophytes, belong 

 to the Molluscans of this family. 



THE FIFTH CLASS OF MOLLUSCA. 



THE BRACHIOPODES.f 



Like the Acephales, the Brachiopodcs have a cloak with two lobes, and this cloak is 

 always ojien. In place of a foot, they have two fleshy arms, garnished with numerous fila- 

 ments, which they can push beyond the shell and withdraw within it : the mouth is between 

 the insertions of the arms. We are not well acquainted with their organs of generation, nor 

 with the nervous system. J They are all covered with a fixed bivalve shell, and are conse- 

 quently destitute of locomotion. We only know three genera of them. 



The Lingul^, Briig. — 

 Have two equal, flattish, oblong valves, with the beaks at the end of one of the narrowest sides, gaping 

 at the opposite end, and attached between the two beaks to a fleshy pedicle, by which they are sus- 

 pended to rocks. Their arms are rolled up spirally, to lie within the shell. It appears that their 

 branchiffi consist of little leaflets, arranged all round each lobe of the cloak, on its internal surface. 

 Only one species (Lhigida anatina, Cuv.) is known, from the Indian Ocean. [Mr. Broderip has described two 



other species.] 



The Tekebratul^, Brug. — 



Have two unequal valves united by a hinge : the summit of one, more protuberant than the other, is per- 

 forated to permit the passageof a fleshy pedicle which attaches the shell to rocks, madrepores, other shells, 

 &c. Internally, a small bony framework is observed, that is sometimes sufliciently complex, composed 

 of two branches, which articulate with the imperforate valve, and which support the two arms, edged 

 all round with long, closely-set fringes, between which there is, on the side next to the large valve, a 

 third simply membranous and much longer appendage, usually spirally convoluted, and fringed like the 

 arms. The mouth is a small vertical fissure between these three large appendages. The principal part 

 of the body, situated near the hinge, contains the numerous muscles, which reach from one valve to the 

 other, and between them are the viscera, which occupy but little space. The ovaria appear to be two 

 ramose productions, adherent to the parietes of each valve. I have not yet been able to satisfy 

 myself in regard to the position of the branchiffi. Numberless Terebratulae are found, in a fossil or 

 petrified state, in certain secondary strata of ancient formations. The living species are less numerous. 



There are some species broader transversely, or longer in the direction perpendicular to the hinge, with a 

 margin entire, or emaiginate, or three-lobed, or with several lobes ; there are even some that are triangular : 

 their surface may be smooth, or furrowed, or veined : they are thick, or thin, or even transparent. In several, 

 instead of a hole in the apex of their valve, there is an emargination, and this is sometimes partly formed by two 

 accessory pieces, &c. It is probable that the animals, when better known, will present generic differences. Al- 

 ready there have been recognized in the 



Spirifer, Sow., two large cones, formed of a spiral thread, which appear to have been the supports of the anirnal. 

 In the Thecidea, Def., the support seems to have been incorporated with the small valve. 



The Orbicul^, Cuv. — 

 Have two unequal valves, one of which, being round and conical, resembles the shell of a Patella : the 

 other is flat, and adherent to rocks. The arms of the animal (Crispus, Poli) are cibated and spirall> 

 curved, like those of the Lingula. 



* On these peculmrities S.ivigny has founded his genera Polpcliniwi, 

 ^plidiiim, Didemnum, Kucalimn, Diazouu, SisHlina, &c., whieli 

 it appears to us unnecessary to preserve. 



t Piilliobnwihiata of M. de BlainviUe. [Rang makes them the Ist 

 order of the Te»t;ieeons Acephales.] 



J [Mr. Owen has an arltnirable memoir on their anatomy in the 1st 

 vol. of the Trans, of the Zonlogicu! Socieli/.] 



§ Observations more precise tlian any we yet have made appear 

 necessary before wc can arrange the Mitgas of Sowcrby. the Strigoce- 

 pliules of Defr.ance, and sonic other groups, near this one. 



