MOLLUSCA, (Lat. mollis,) Malakia and Ostrocoderma of Aristotle; ltiollusques Fr.; Weichthiere, Mollusken, Germ.; Mollusks, Engl.; invertebrated animals, distinguished by Bruguiere from insects by the negative cha racters of the absence of bones, of stigmata, and of jointed feet ; and first accurately de fined by Cuvier as a primary division of the animal kingdom, with the following anatomical characters. The Mollusks are animals without an articulated skeleton or a vertebral column. Their nervous system is not concentrated in a spinal marrow, but merely in a certain number of medullary masses dispersed in different points of the body, the chief of which, termed the brain, is situated transversely above the oesophagus, nd encompasses it with a nervous collar. Their organs of motion and of the sensations have not the same uniformity as to number and position as in the Vertebrata, and the irregularity is still more striking in the viscera, particularly as respects the position of the heart and respiratory organs, and even as regards the structure of the latter : for some of these respire elastic air, and others water, either fresh or of the sea. Their external organs, however, and those of locomotion are generally arranged symmetrically on the two sides of an axis. The circulation of the Mol lusks is always double; that is, their pulmo nary circulation describes a separate and dis tinct circle. The blood of the Mollusks is white or bluish, and it appears to contain a smaller proportionate quantity of fibrine than that of the Vertebrata. The veins probably fulfil the functions of absorbent vessels. Their muscles are attached to various points of their skin, forming more or less dense and complex tis sues. Their motions consist of various con tractions, which produce inflections and pro longations of their different parts, or a relax ation of the same, by means of which they creep, swim, and seize upon various objects, just as the form of these parts may permit, but as the limbs are not supported by articu lated and solid levers they cannot advance rapidly. Nearly all Mollusks have the body enveloped by a duplicature of soft and usually muscular integument, which bears more or less resemblance to a mantle: it is sometimes nar rowed into a simple disc, sometimes prolonged into one or more tubes, or is extended and divided in the form of fins.
The eked Mollusks are those in which the mantle is simply membranous or fleshy. In most of the species one or more plates, of a substance more or less hard, are developed in its substance, usually in successive layers, which increase in extent as well as in thick ness as they are successively formed. When this substance remains concealed in the thick ness of the mantle, it is still customary to style the animals ' Naked Mollusks.' Most com monly, however, it becomes so much deve loped that the contracted animal finds shelter beneath or within it, and it is then termed a shell, and the animal is called a Testaceous Mollusk.
The mode of generation is too varied in the Mollusks to afford any common character to this sub-kingdom : but, being for the most part sluggish and feeble animals, with low and little varied instincts, they are preserved chiefly by their fecundity and vital tenacity.
The nervous system of the Mollusks may consist of one or more ganglionic masses ; but these are scattered, often irregularly or unsym metrically, through the body, and the inter communicating chords never form a symme trical knotted pair along the middle line of the ventral surface of the body ; whence the term lielerogangliata; expressive of the charac teristic condition of the nervous system, with which the unshapely and often unsymmetrical figure of the whole body corresponds.
The number of the ganglia follows closely the progressive development of the muscular system: the first is that which is found between the anal and respiratory tubes of the Ascidix, and which regulates the elongator and sphincter muscles of these tubes. The development of a muscular heart and of a bivalve shell with its adductor muscle is accompanied with the appearance of a second ganglion or centre of the nerves which supply that muscle: a second adductor muscle, with the superaddition of a muscular foot and its retractors, produces an additional ganglion or ganglions. and the com plication of the nervous system is further aug mented when the breathing and anal siphons are unusually prolonged, and provided with strongly developed annular contractile muscles and with proportionately powerful retractors. The progressive development of the nervous system may be traced thus far in the tunicated and bivalve Mollusca without its reaching the stage which is marked by the appearance of a distinct supra-cesophageal ganglion or brain . the species have, in fact, no distinct head, and are termed Acephalous Mollusks. The first appearance of this important part with its appendages, which are usually subservient to the organs of special sense, is associated with an accumulation of nervous matter, in the form of a transverse chord, with a ganglion or gang lions, above the commencement of the ceso phagus, forming the brain; whence these Mol lusks are termed Encephulous.' The development of the brain proceeds directly as the organs of the senses increase in number and complication, and consequently reaches its maximum in the higher Cephalo pods with highly complex eyes and distinct organs of hearing; and in these the brain is protected by a cartilaginous cranium, forming the first representative of the true internal skeleton which is met with in the Invertebrate division of animals, and one of the main organic characters by which the higher Mol lusks surpass Articulates in the ascent to the Vertebrate type.