Page:The American Cyclopædia (1879) Volume V.djvu/203

 COtfCHIFERA CONCHOLOGY 199 in variety and importance, and are invariably aquatic. They are all inhabitants of the sea, excepting a few widely scattered genera, live on every coast, and are found in every climate. The shells of the bivalves are united at the back by an elastic ligament and articulated by a hinge, which is sometimes furnished with teeth shutting by the side of each other and acting much like the common butt hinge. The valves are closed by strong muscles which pass from one valve to the other, and when these are re- laxed the shells open spontaneously by the contraction of the cartilage. After the death of the animal, when the muscles lose their power, the elasticity of the ligament causes the valves to gape wide, more so than during the life of the mollusk. (See CONOHOLOGY.) The mantle, which is a conspicuous part of the animal, is a broad membrane which lines all the interior of the shell and encloses the whole body. Its edges are more or less fringed, and are either free, partly united, or entirely so, ex- it. Mussel. 2. Nucula. Venus verrucosa. anatinus. Anodon cepting a passage for the foot before and for the siphons behind. The foot is a muscular mass which may be protruded from the shell, and serves as the organ of motion. In nucula and some others the foot is deeply cleft, and capable of expanding into a disk like that on which the snail glides ; while in the mussel, pearl oyster, and others, which habitually spin a byssus or thread of fibres by which they attach themselves permanently to substances, the foot is finger- like and grooved, and serves only to mould and fix the threads of which the byssus is composed. The branchiae or gills are arranged somewhat like ruffles behind the foot, enveloping the ab- dominal mass. These are the respiratory or- gans, but they serve also another and very im- portant purpose. Firmly fastened to rocks and other substances, or at best moving slowly and awkwardly in their muddy or sandy beds, these animals have not the power of following their prey, nor are they furnished with the means of seizing it, but the branchiae convey to the mouth whatever particles the current brings, whether organic or inorganic, animal or vegetable ; upon these the mollusk lives. The water is furnished to the branchiae by one siphon, while another serves as a passage for the excrement. The branchial siphon has its orifice surrounded by a double fringe. When unmolested, a current flows steadily into the opening of this siphon, while another current rises up from the exha- lant tube. The burrowing species have a strong foot, with which they bore into the sand and clay upon the shore so as to entirely conceal themselves. They never leave these abodes, and often become fossilized in them. The tere- do or ship worm, and some other borers, which were formerly included among the univalves and multi valves, are now arranged in this class. The interior of the shell is marked with char- acters derived immediately from the shellfish, and affording a surer clue to its affinities than those which the exterior presents. The struc- ture of the hinge characterizes both families and genera, while the condition of the respiratory and locomotive organs may be to some extent inferred from the muscular markings. CONCHO, a W. county of Texas, bounded N. E. by the Colorado river, W. by Bexar district, and intersected by the Rio Concho and other streams; area, 1,025 sq. m. ; yet unsettled. The surface is broken and rocky. Timber is scarce. The climate is dry and salubrious. CONCHOLOGY (Gr. n6-y X V, a shell, and Uyos, a discourse), the science which treats of the tes- taceous parts of the mollusca. Formerly these were made the basis for classifying the animals to which they belong ; but the enlarged study, introduced by Cuvier, of the anatomical struc- ture of the bodies contained within the shells, has developed an apparent want of corre- spondence between the latter and the organiza- tion of the more important parts, upon which the habits and true character of the animal must mainly depend. (See MOLLUSCA.) In the old classification, molluscous animals possessed of the same peculiarities of internal structure were widely separated by the differences of their external covering ; some animals, as the cirripedes, comprising nearly all the old divi- sion of multivalves, were included with the mollusca because of their shelly covering, and thus they continued until their structure at last was found to refer them to the articulata ; and some mollusca possessed of no shell, as the nudibranchs, would properly be excluded from the arrangement that comprised others hardly differing from them, except in being provided with this appendage, or perhaps but a poor apology for it. And then, as some species have the faculty of leaving and returning to their shells, it might occur, in ignorance of the rela- tions of each part to the other, that the shell should be referred to one department and the body to another of the animal kingdom. In- deed, by reason of the changes of form and color that take place in the growth of some shells, the same species have been described at different ages of the individual under different names.