364 



MOLLUSCA. 



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 baked 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 

 ' Heterogangliata,' 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 Ascidise, 

 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 ^lollusks. The first 

 appearance of this important part with its 

 appendages, which are usually subservient to 

 the organs of special sense, is associated witli 

 an accumulation of nervous matter, in the form 

 of a transverse chord, with a ganglion or gang- 

 lions, above the commencement of the oeso- 

 phagus, forming the brain ; whence these Mol- 

 lusks are termed ' Encephalous. 1 



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. 



The nervous system of the Mollusks is re- 

 markable for the peculiar and distinct colour 

 of the ganglions in certain species, as the fresh- 

 water Muscles and Unios ; and for the contrast 

 between the density of the cellular sheath of 

 the nerves and the semifluid pulp which it 

 contains. This structure allows of an artificial 

 injection of the nerves, which has led some 

 anatomists to describe them as parts of an 

 absorbent system. 



Although the bivalve Mollusks are headless 

 and without brain, some of them have mani- 

 fested signs of a perception of light, and in a 

 few species simple ocelli have been detected on 

 the verge of the mantle. 



In the Encephalous Mollusks the eyes, 

 when present, are small, never exceed two in 

 number, and are usually supported on flexible 

 peduncles or tentacula; but in the Dibran- 

 chiate Cephalopods they are large, always ses- 

 sile, and highly complicated. 



The organ of hearing is peculiar to the Ce- 

 phalopods in the present division of the animal 

 kingdom. 



The organ of smell, as a special and cir- 

 cumscribed part, has likewise been recognized 

 only in the highest class : but Cuvier observes, 

 that the skin" of the Mollusks so clearly re- 

 sembles in its softness and lubricity a pituitary 

 membrane, that they probably may recognize 

 odours at every point of their external surface. 

 Professor De Blainville conjectures that in the 

 Gastropods the soft extremities of the first pair 

 of cephalic tentacles may be the seat of the 

 organ of smell. An organ of taste has of 

 course been recognized only in the Encephalous 

 Mollusks, in many of which the tongue is large 



