This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1191). The term Mollusca, employed by Cuvier to designate the fourth grand division of the animal world, is obviously derived from a very unimportant circumstance of their organization, which the tribes included in it possess in common with innumerable forms of very dissimilar beings, whose soft bodies are unsupported by any internal or tegumentary framework of sufficient density to merit the name of a skeleton. Subsequent anatomists have therefore, however unwillingly, been compelled to substitute another name for that given by the illustrious French zoologist to this extensive class, the boundaries and relations of which, as at present admitted, remain precisely as they were first established by his patient and unwearied investigations relative to the anatomical structure of the animals comprised within its limits.
(1192). It is to the arrangement of the nervous system that we must again have recourse in order to discover a distinctive appellation; nor in this shall we be disappointed; for here we at once find a character peculiar to this great section of animated nature, and generally applicable to the various classes composing it. All the Mollusca present nervous ganglia, which, in the more highly organized forms, attain considerable development and consequent perfection; but these nervous centres, instead of being arranged in a longitudinal series of symmetrical pairs, are variously distributed in different parts of the body, - an arrangement exactly correspondent to the want of symmetry observable both in the external configuration of these creatures and in the anatomical disposition of their internal viscera. Still, however, one large ganglionic mass occupies a position above the oesophagus, and it is with this that the nerves of the existing senses invariably communicate; so that we are naturally induced to regard this as the sentient brain, corresponding with the supra-oesophageal ganglion of the Arti-culata both in position and office.
The other ganglia vary considerably both in number and in situation; but, wherever placed, they all communicate with the supra-cesophageal mass, while the branches derived from them are distributed to the viscera, or to the locomotive organs.
(1193). Various are the forms and widely different the relative perfection of the Mollusca, as regards their endowments and capabilities. Some, as the Polyzoa, fixed to the surface of foreign bodies, either immoveably or by the intervention of a flexible pedicle, entirely deprived of organs connected with the higher senses, and unable to change their position, are content to cast out at intervals their ciliated arms, which form a net of Nature's own contrivance, and thus entrap such passing prey as suits their appetite. Others, equally incapable of locomotion, but furnished with arms of different construction (Brachiopoda), catch their food by similar efforts. The Tunicata, enclosed in coriaceous bags, are firmly rooted to the rocks; or, aggregated into singular compound masses, float at the mercy of the waves. The Conchifera inhabit bivalve shells; while the Gasteropod orders, likewise defended in most cases by a shelly covering, creep upon a broad and fleshy ventral disk, and, thus endowed with a locomotive apparatus, exhibit senses of proportionate perfection.
The Pteropoda swim in myriads through the sea, supported on two fleshy fins; while the Cephalopod Mollttsca, the most active and highly organized of this large and important division of animated nature, furnished with both eyes and ears, and armed with formidable means of destroying prey, become tyrants of the deep, and gradually conduct us to the most exalted type of animal existence.
(1194). These different sections, which constitute, in fact, so many distinct classes into which the Heterogangkliata have been divided by zoologists, we shall now proceed to examine seriatim; beginning, as heretofore, with the most imperfectly organized, and gradually tracing the development of superior attributes and more exalted faculties as the nervous centres attain greater magnitude and concentration.