THE MUSEUM 



NATURAL HISTORY 



ZOOLOGY. 



Sub-kingdom — MOLLUSC A* — Molluscous Animals. 



If we follow the anangemenl of the animal kingdom, 

 as exhibited by the immortal (Juvier, we find that 

 animals are constructed upon four different types, which 

 have been called Suh-kinijdoms. The first of tliese is 

 the sub-kingdom Vertehrata, or vertebrated animals, 

 and has occupied the former part of this work. The 

 second sub-kingdom is that of the MoUusca, composed 

 of those animals that are generally known by the name 

 of Shells or Shell-fish. Formerly the animals which 

 formed and inhabited shells were httle known, and by 

 those who studied this interesting branch of natural 

 history, were very little attended to. The shells or 

 coverings of the animals alone attracted the attention 

 of the student, and hence the study of this branch of 

 natural science received the name of Conchologyf — an 

 appellation still in general use. At the present day, 

 however, the animals receive an equal share of atten- 

 tion from the naturalist ; and the arrangement of the 

 hard persistent coverings, or shells, of these animals, 

 depends almost entirely upon the structure of the soft 

 animals which form them. We find, too, that a large 

 number of animals which are true shell-fish, so to 

 speak, have no shells, and hence the name oi Molltisca 

 now adopted to contain all, both shell-bearing and 

 shell-less species. The Greeks, as Aristotle and his 

 followers, called them by a similar name, Malalia 

 (iaa>.a/t/a), and the study was hence by several natur- 

 alists called Malacology. 



Molluscs may be defined as animals possessing a 

 body which is soft and fleshy, devoid of bones or any 

 internal skeleton, and not divided, as in insects and 

 worms, into rings or articulations. This body is 

 covered with a soft, irritable, and muscular skin, which 

 is moistened by a viscid liquor that exudes from it, 

 and which is in very many instances of sufficient extent 

 to form fleshy folds that envelope, more or less com- 

 pletely, the whole body as in a mantle or cloak. In 



• From the Latin word mollis^ soft. 



t From the Greek words Koy-^ri, a shell, and Xoyot, ft dis- 

 course. 



some cases this skin is naked, and then the mantle is 

 thick and mucous ; in the greater number of the species, 

 however, it is protected by a hard, persistent, calcare- 

 ous covering called a shell, and then this mantle is 

 thin and transparent. The animal, in the great majority 

 of species, is of an elongate form, walks upon a central 

 disc ox foot, and is provided with one or more pairs of 

 organs on the head and sides. The principal organs 

 of the body are in pairs and are symmetrical, thus dis- 

 tinguishing them from the Radiated sub-kingdom heie- 

 after to be mentioned. But the most essential character 

 lies in the arrangement of their nervous system. This 

 system consists of a certain number of enlargements 

 or centres, called ganglia, from which the nerves are 

 given off to the different parts of the body. These 

 ganglia are principally concentrated around the entrance 



Fig. 209. 



Nervous system of MoUusca. 



to the alimentary canal, and form a collar or ring 

 which surrounds the asophagtis or throat, and is con- 

 nected with other ganglia disposed without symmetry 

 among the viscera, or in the neighbourhood of the 

 organs of locomotion — fig. 209. The ganglia " on the 

 sides and upper part of the ring represent the brain, 

 and supply nerves to the eyes, tentacles, and mouth ; 



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