582 



SUPPLEMENT. 



ORDER V.- PTEROPODA. 



Destitute of any shell, foot or arms, cither for loco- 

 motion or seizing their prey; but provided with two 

 opposite and similar fins, fitted for swimming. The 

 animals of this order are provided with two opposite 

 fins; they are mostly of a small size, and either desti- 

 tute of appendages, or having very short ones, situate 

 at the head; some species are provided with a thin 

 cartilaginous or horny shell. Hyalaa tridentata, pi. 24, 

 f. 3ti. 



CLASS II. 



ACEPHALOUS MOLLUSCA, OR SOFT ANIMALS DESTITUTE 

 OF A HEAD. 



The second great division of testaceous Mol- 

 lusca, consists of animals which are destitute of 

 any apparent head, and have been denominated 

 by Lamarck, CONCHIFERA. Their mouth is con- 

 cealed in the bottom, or between the folds of 

 their mantle, which is usually doubled and 

 incloses the body, in the same manner as a 

 book is clasped over by its cover ; but it often 

 happens, that, in consequence of the two lobes 

 uniting before, it forms a tube ; sometimes it 

 is closed at one end, and then it represents a 

 sac. The mantle is generally covered by a 

 calcareous, testaceous bivalve shell united by 

 a hinge or ligament, and in some instances 

 the shell is multivalve ; in two genera, how- 

 ever, it is only of a cartilaginous or membrana- 

 ceous nature. The animal is always attached 

 (o this shelly covering by strong muscles, by 

 means of which it can open or close the sides 

 of its covering at pleasure. The brain is 

 situated over the mouth, where one or two 

 other ganglia are observable. The branchiae 

 or gills, generally consist of large lamellas 

 covered with vascular meshes, under or between 

 which passes the water ; and are more simple 

 in those genera which are destitute of a shell. 

 The leaflets of which the branchiae are com- 

 posed are generally crescent-shaped, placed 

 on each side of the cloak, covering the abdomen 

 of the animal upon the sides of which they are 

 attached in pairs ; they are formed of a tissue 

 of small vessels arranged close together, like 

 the pipes of an organ. From these branchiae 

 the blood proceeds to the heart, which is placed 

 towards the back, is very small, but provided 

 with venous and arterial vessels. The liver 

 is extensive, and embraces the stomach and 

 the greater proportion of the alimentary canal. 

 The mouth is always devoid of teeth, arid is 

 capable of only receiving the molecules, con- 

 ducted to it by the water ; at each side of the 

 mouth are four thin triangular leaflets, which 

 are the extremities of the lips. Some species 

 are provided with two stomachs. The whole 

 animals of this division fecundate themselves, 

 and in many of the species, the young ones 

 are extremely numerous, and pass some time 

 in the thickness of the branchiae before quitting 

 the parent. The whole acephala are aquatic 



animals. The foot of the animal when it 

 exists is a mere fleshy mass, the motions of 

 which are effected by a mechanism analogous 

 to that which acts on the tongue of the mam- 

 malia. 



The Conchifera of Lamarck are separated 

 into two orders as follows : 



ORDER I. MONOMYARIA. 



With but one internal muscle of attachment, and the 

 shell marked interiorly with one subcentral muscular 

 impression. 



SECTION I. 



Ligament none, or at least unknown, or replaced by a 

 tendinous cord, which supports the shell. 



FAMILY I. BRACHIOPODA. 



With two opposite ciliated arms, placed near the 

 mouth, rolled up in a spiral form when the animal is in 

 a state of repose ; mantle provided with two separate 

 lobes before, which envelope the body. Shell bivalve, 

 adhering directly, or by means of a tendinous cord, to 

 marine substances. Lingula anatina, pi. 32, f. 1. 



FAMILY II. RUDISTA. 



Nothing is known of the animal, nor of the ligament 

 and hinge of the shell; the valves are very unequal; and 

 no distinct hooks are visible. Crania personata, pi. 24, 

 f. 75. 



SECTION II. 



Ligament not marginal, but inclosed in a hollow space 

 under the beak, always perceptible, and never forming 

 a tendinous cord under the shell. 



FAMILY I. OSTRACEA. 



Ligament interior, or at least partly so; shell irregular, 

 foliaceous, and sometimes papyraceous. The animal 

 has neither foot, arm, nor projecting siphon. Anomia 

 epkippium, pi. 24 3 f. 41 . Ostrea Edulis, the Edible oyster, 

 pi. 24, f. 73. f. 98, muscle of the oyster. 



FAMILY II. PECTENIDES. 



Ligament either interior, or partly so, being discov- 

 erable in some through an opening in the joint; shell in 

 general regular, compact, and not foliaceous. Some 

 genera fix themselves to marine bodies, by a byssus, 

 while in others the lower valve is immovably attached; 

 and many of the pectens are at liberty. Sjxmdylus 

 qceder&pus, pi. 24, f. 52. d e, the branchiae; b, the mouth; 

 'g h e,the abdomen; &, the great adductor muscle. Pecten 

 'Jacobaus, pi. 24, f. 77. a b c d, the two lobes of the 

 mouth; e e, the byssus; //, the abdomen; z, the intes- 

 tinal canal; 9, the mouth, ornamented with a fringe; //, 

 the great adductor muscle. The animals of this genus 

 possess considerable powers of locomotion, which was 

 well known at the time Aristotle wrote. They have the 

 power of removing themselves from one place to another, 

 on dry land, by successive leaps. This has been fully 

 established by recent experiments. A basket filled 

 with the Pecten opercularis, or common edible scallop, 

 has been placed on the sea shore, near the edge of the 

 water, when all the individuals have speedily found 

 their way to the sea. Lima squamosa, pi. 24, f. 72. 



SECTION III. 



Ligament marginal and elongated. 



FAMILY I. MALLEACEA. 



Ligament marginal, sublinear, and either simple or 

 interrupted by crenulations ; shell foliaceous or more 

 or less ineqinvalve. Avicula Tarcntimt, pi. 24. f. PO, 

 abed, the mantle, the duplicate of which contains the 



