356 STRUCTURE OF THE MOLLUSCS. 



the principal characteristics which designate these, beings, and separate them equally 

 from the apparently less organized forms above them and the apparently more organized 

 forms below. 



In the first place, it is worthy of notice, that except in the cephalopods, popularly 

 Known as the cuttle-fishes, which are thought by many anatomists to form a distinct class 

 by themselves the symmetry of arrangement which has been found throughout the 

 previous order is now seen no more ; the corresponding pairs of limbs and other 

 structures now vanish, and the only indication of its existence is found by close examina- 

 tion of the nervous system. 



There is no true brain in these animals, the whole nervous system being composed of 

 certain masses, or knots of nervous matter ; from which radiate the needful filamentous 

 conductors of the mysterious life-power. The visual organs of the Molluscs are, with a 

 few exceptions, but slightly developed, and are far inferior to those of the crustaceans, 

 insects, and other creatures which take a comparatively low rank in the animal kingdom. 

 Many Molluscs are clearly destitute of these organs when adult, though on their primary 

 entrance into life they are gifted with a tolerably well developed, though evanescent organ 

 of sight. Whenever the true eyes are present, they are only two in number, and are 

 sometimes placed in very singular situations, the well-known black-dotted tentacular eyes 

 of the common snail affording a familiar example. The little coloured points which exist 

 in some species are not here taken into consideration. 



The power of progression in the Molluscs is greatly varied, being in some species 

 almost absent, while in others it is developed to a wonderful degree. Many of these 

 creatures, such as the mussel, the limpet, and the oyster, scarcely stir from the spot where 

 they have once fixed their habitation ; the snail and those of a similar form glide slowly 

 along by means of the curiously developed mass of muscular fibres, technically called, 

 from its use, the foot ; the scallop drives itself through the water in short jerks or flights, 

 caused by slowly opening and then rapidly shutting its valves ; several species are known 

 to jump by a sudden stroke with the foot; the nautilus urges its shell through the 

 tfaves by the violent expulsion of water from its interior, and is driven along on just 

 the same principle by which a sky-rocket soars into the air ; and the flying squid, one of 

 the cuttle-fishes, is able to rival even the flying-fish in its aerial journeys, shooting 

 through the air to considerable heights, and even leaping fairly over both bulwarks of a 

 ship and alighting in the water on the opposite side of the vessel. 



The old fable of the nautilus and its sails has long been rejected, but the fabricators 

 of this legend need not have visited the ocean for an example of a molluscan boat. 

 Any one who is in the habit of watching the streamlets that irrigate while they drain 

 our meadow lands, must have seen the common water-snails come floating down the 

 current, lying on their backs, their shells submerged, and the edges of their fleshy foot 

 turned up on all sides so as to convert that organ into a miniature flat boat. 



That the Molluscs, or at all events some of the species, possess the sense of hearing, is 

 tolerably evident from an examination of the structures. Near the nervous knobs, or 

 ganglia as they are scientifically termed, of the head, are placed some little vesicles, each 

 filled with a transparent fluid, and containing a tiny knob or spikelet of chalky matter, 

 very similar to the well-known ear-bones of fishes, and probably serving a similar 

 purpose. These " bones " appear to be perpetually in motion within their crystalline cell. 



The circulation of the Molluscs is tolerably defined, especially in the higher and best- 

 developed species, where the blood is urged on its course by a definite heart, and ramifies 

 through the body by means of well-developed vessels. In the lower forms, however, these 

 vessels can no longer be distinguished, and the blood circulates through a system of little 

 cavities distributed in the body. So completely is this the case, that many Molluscs can 

 be successfully injected, by introducing a fine-nosed syringe at random into the body, and 

 pressing the heated substance very gently into the system. 



The body is surrounded by a soft membrane termed the mantle, which is flexible 

 extensile, and contractile to a marvellous degree. The movements of the Molluscs are 

 mostly performed by means of the mantle, and through this structure the shell is secreted 

 and moulded into form. 



