418 CONCHIFEKA. 



cilia, and by means of the constant action of the cilia a current is caused, which drives 

 a continual stream over the mouth, and enables the animal to seize the minute animals 

 that dwell in the sea and are distributed throughout the waters. 



WE will now proceed to the examination of our selected examples of these curious 

 molluscs. 



"With the exception of the elongated shells or footstalks, and the dark shell in the 

 centre, all the specimens shown in the illustration belong to the genus Terebratula, 

 this name being derived from a Latin word signifying a wimble, and given to the 

 animal in allusion to the round hole which perforates the beak. The popular name of 

 LAMP-SHELL also refers to the same aperture, because it looks like the round hole through 

 which the wick of an ancient lamp is drawn. The structure of the shell itself is very 

 curious, being made up of innumerable flattened prisms laid side by side and arranged in a 

 slightly oblique position, so that their ends project over each other, something like the 

 slates in a house-roof. The substance of the shell is also perforated by multitudes of 

 very minute circular apertures. 



BETWEEN the Yellow Lamp-shell and the empty valve may be seen the PARROT-BILL 

 LAMP-SHELL, so called from the shape of the beak, which is long and hooked in a manner 

 which much resembles the beak of the bird whose name it bears. The colour of this species 

 is black. 



OUR last example of these remarkable molluscs is the GOOSE-BILL LAMP-SHELL, seen 

 on the extreme right of the engraving. All the members of the family to which this 

 animal belongs are known by the long and comparatively narrow valves, and the foot- 

 stalk which attaches them to the rocks, and which passes from between the valves. The 

 substance of the shell is rather soft and perforated. The valves are slightly open at each 

 end, and blunted in front. Very little is known of its habits in the living state, but it is 

 worthy of notice that the Goose-bill Lamp-shell is the oldest known form of organic life. 



THE next great group of molluscs is that which is known by the technical term of 

 Conchifera, and includes all the ordinary bivalves, i. e. where each valve corresponds with 

 the right or left side, and not with the upper and under surface, as in the brachiopodsjust 

 described. 



Though not possessing so many species as the gasteropoda, this group surpasses it in 

 point of numbers, the bivalves being produced in countless myriads, and, perhaps, less 

 exposed to the attacks of foes than most of the race. They are extremely useful in both 

 salt and fresh water, feeding on the particles that would otherwise pollute the element in 

 which they live. Their mode of feeding is somewhat similar to that of the last-mentioned 

 group, the water being driven over the mouth by the continual action of certain appen- 

 dages, and there cleared of all its solid portions. So completely does a bivalve effect this 

 purpose, that it even intercepts the microscopic plants and animals which are invisible to 

 the naked eye, and conveys them to the stomach with marvellous certainty. 



If the stomach of a scallop, for example, be opened, and its contents touched with 

 nitric acid and heat, myriads of flinty skeletons will be found in it, once supporting the 

 minute structures of diatoms and other organisms well known to any microscopist. Even 

 if some water be coloured with indigo, and a healthy bivalve placed in the same vessel, 

 the water will gradually lose its dark blue tint, and finally become clear and colourless, 

 the particles of indigo having been removed by the mollusc as the water is driven over 

 the gills and mouth. 



The whole structure of a bivalve is very curious, and can be readily studied by any 

 one who chooses to purchase a few oysters, or mussels, or cockles. Those parts of the 

 structure which are of most importance will be briefly described while treating of 

 individual species. 



Systematic naturalists are rather perplexed with regard to the particular characteristics 

 which must be employed in the arrangement of the molluscs ; and the necessary con. 

 sequence of this uncertainty is, that almost each investigator has invented a fresh system, 



