666 MOLLUSCA. 



structure may be detected on the surface; and this is frequently so thick as to make up a great part ot' 

 the shell, as in Pinna, Avlcula, and a large proportion of Cuvier's family Ostracece. The internal 

 layer is usually more compact, and presents less distinct traces of cellular structure. In the shells of 

 the Cuvierian families, Camacece, Cardiacea, and Inclusa, the greater part of the thickness is formed 

 by the internal layer. In the Mytilacecc, both layers are usually well seen, the inner layer, however, 

 being usually the thicker. The successive additions to the shell are not made on the same plan in the 

 two layers. The outer layer is merely extended by the junction of an additional portion to its margin; 

 but the inner layer receives an addition to its whole internal surface, so that its thickness is increased, 

 as well as its extent. This is well seen in the common Oyster, in which the successive layers of the 

 shell remain unusually distinct. The white inner portion, of which the greater part of each valve is 

 composed, is made up of a number of laminae, each of which extends beyond the one external to it ; 

 and thus the outer laminse are at the same time the oldest and smallest. Each of the layers of this 

 substance is covered at its edge by an elastic yellowish-brown margin, which is so arranged that if 

 the successive layers were closely adherent to each other, this substance would form a complete ex- 

 ternal covering to the shell. A section of the shell of an Unto, in which the layers do thus adhere, 

 shows that the mode of growth of the two layers of a compact shell is essentially the same. 



In many Bivalve shells of the class Brachiopoda, especially belonging to the genus Terebratula 

 and its allies, a very curious arrangement exists; the shell being perforated by a number of minute 

 apertures, intending from the internal to the external surface; and these canals being filled with 

 prolongations of the soft tissue of the animal itself. 



A number of curious varieties of shell-structure have been described by Dr. Carpenter (loc. cit.) ; 

 who has also shown that, in many families and genera of bivalves, the structure of the shell affords 

 characters of great importance in classification; and that it is possible in several instances to recog- 

 nize the family, or even the genus, to which a specimen belonged, by the microscopic examination of 

 but a very minute fragment of it. 



In the Univalve shells formed by Mollusca of the class Gasteropoda, the arrangement, is generally 

 different, especially in the porcellanous shells of many of the Pectinibranchiata. These are of great 

 density, and contain very little animal matter. 1 hey have three layers instead of two, and these three 

 layers are similar to each other in structure, being composed of a series of rhomboidal plates, disposed 

 vertically to the surface of the shell, and giving an appearance of crystalline structure when the shell is 

 broken across. The direction of these plates is the same in the inner and outer layers ; but it is 

 transverse in the middle layer, by which arrangement a greatly increased strength is gained. Each 

 plate is made up of a set of long narrow cells filled with carbonate of lime, and adherent to each other 

 at their edges. The shells of Gasteropoda do not, for the most part, undergo any thickening by suc- 

 cessive additions; the enlargement required by the growth of the animal being effected by the exten- 

 sion of the margin, with no further addition to the previous internal layer than is sufficient to give it 

 the requisite smoothness along the line of junction of the new and old portions. 



In the General Classification of the Mollusca, given by Cuvier, the chief alteration required by the 

 progress of knowledge is the entire removal of the Cirrhopoda to the sub-kingdom Articulata ; their 

 affinity to which, perceived by Cuvier, has been since placed beyond doubt by the discovery, that they 

 issue from the egg in a condition resembling that of certain low forms of Crustacea, having eyes and 

 active powers of locomotion, and only acquiring the form and condition of Cirrhopods after a series of 

 metamorphoses, in which the eyes are lost, the locomotive organs altered in character, and the shell 

 formed. This shell, in the Balanus and other sessile Cirrhopods, is composed of an outer and an 

 inner plate, separated by adiploe or cancellated texture, which is sometimes of considerable thickness. 



Perhaps the best primary division of the true Mollusca is into the encephalous, or those provided 

 with a head, and the acephalous, or headless ; in which last the mouth is not placed on a prominent 

 portion of the body, but is concealed by the projection of the mantle. 



The first division includes the three classes of Cephalopoda, Pteropoda, and Gasteropoda, 

 from which last the Heteropoda are detached by some Zoologists, to be ranked as a distinct class. 



The second division may also be arranged into three classes, namely — the Conchifera, forming 

 the testaceous division of the Acephala of Cuvier; the Brachiopoda; and the Tunic ata, forming 

 the naked or shell-less division of the Acephala of Cuvier. The first two of these classes both possess 

 bivalve shells ; but the structure of the animal is very different, the respiration being carried on in the 



