602 



MOLLUSC A. 



appears in the form of ganglia and filaments. The 

 principal ganglion, or the one to which the term brain 

 is usually applied, is seated above the gullet or entrance 

 to the stomach. It sends out nerves to the parts about 

 the mouth, to the tentacula, and to the eyes. It may 

 be considered as analogous to the cerebrum of the ver- 

 tebral animals. From this ganglion proceed two fila- 

 ments, one on each side, which in their descent inclose 

 the gullet, and unite underneath to form a second gan- 

 glion. From this last, which has been compared to the 

 cerebellum, numerous filaments are likewise distributed 

 to the parts around the mouth, and to the other regions 

 of the body. These filaments in some cases again 

 unite, and form subordinate ganglia. In many cases 

 the brain and ganglia are of a white colour, and gra- 

 nulated structure ; while the nerves which issue from 

 them are white and uniform. The covering of the first 

 ganglion, which is analogous to the dura mater, does 

 not adhere to it closely, but leaves a space filled with 

 loose cellular matter. The tunics of the nerves are 

 equally detached ; and as they can be inflated or in- 

 jected readily, they have led some to suppose that the 

 nerves were hollow, and others, that the tunics were 

 the vessels of the lymphatic system. 



The organs of perception common to the higher 

 classes of animals do not all exist in an obvious man- 

 ner among the mollusca. The loach, that universal 

 sense, is here displayed in many cases with great deli- 

 cacy; and the tentacula, and other cuticular elongations 

 which we have already referred to, contribute to aug- 

 ment its resources. The sense of sight is by no' means 

 universally enjoyed by the inhabitants of this class. In 

 a few species the eye is constructed on the plan of the 

 same organ in the vertebral animals. In general, how- 

 ever, it appears only as a black point, whose peculiar 

 functions can only be inferred from analogy. In many 

 animals there is no trace of an eye, consequently they 

 cannot possess that varied information which the high- 

 er animals obtain from that organ. Where eyes exist 

 in this class, they are uniformly two in number. In 

 one tribe only, namely the cuttle-fish, have the rudi- 

 ments of the organs of hearing been detected. The or- 

 gans adapted to smelling cannot be exhibited, but the 

 existence of the sense is demonstrated by the facility 

 with which they discover suitable food when placed 

 within their reach. The sense of taste exists, but it is 

 difficult to point out the particular parts of the mouth 

 fitted for its residence. But as they select particular 

 articles of food in preference to others, we reasonably 

 conclude that taste regulates the choice. 



It is difficult to form an accurate estimate of the 

 knowledge of external objects which molluscous animals 

 can obtain by means of their organs of perception, join- 

 ed to their powers of locomotion. The kind of life 

 which they enjoy is so widely different from our own ; 

 and attempts to tame them, or vary their habits by edu- 

 cation, are limited by so many circumstances, that we 

 must ever remain in ignorance of their mental powers 

 or capabilities. M. Lamarck ranges them among his 

 Animeaux Sensibles, and considers that, by means of 

 their sensations, they acquire only perceptions of ob- 

 jects, or simple ideas, which they are incapable of com- 

 bining by any mental process. Had this opinion rest- 

 ed on observations, or even on arguments, it might 

 have been worth while to controvert it, but, as it forms 

 a part of the metaphysical speculations of that zoolo- 

 gist, built upon gratuitous assumptions, it requires no 

 farther notice. 



In the classification of the mollusca, the characters 



furnished by the nervous system, from the difficulty of MoiioiM. 

 their detection and exhibition, have never come into S "~V^" 

 use. But those furnished by the organs of perception 

 are highly prized. Of these, the eye is the most ob- 

 vious and constant. It varies in position in different 

 species ; but among individuals of the same species its 

 characters are constant. 



5. Digestive System. In the cutaneous, muscular and Digestir* 

 nervous systems, traces of a general plan may be observ- y telB - 

 ed, according to which they have been constructed in 

 the different tribes. In the organs which remain to be 

 considered, there is less uniformity of structure, each 

 family, almost, being constructed according to a model 

 of its own. 



The time when molluscous animals feed has not 

 been carefully attended to. Those which live in the 

 water are beyond the reach of accurate observation. 

 Those that reside on land usually shun the light, and 

 creep forth in the evenings to commit their depreda- 

 tions. During warm dry weather, they stir not from 

 their holes. 



The animals under consideration feed equally on the 

 products of the vegetable and animal kingdom. Those 

 which are phytivorous appear to prefer living vege- 

 tables, and refuse to eat those which are dried. We are 

 not aware that putrid vegetable matter is consumed by 

 them, although many of the snails and slugs are found 

 under putrid leaves and decayed wood. In these 

 places there' is shelter from the sun, together with 

 dampness, so that it is difficult to determine whether 

 they sojourn in an agreeable dwelling, or a well-stored 

 larder. Those mollusca which are carnivorous prey on 

 minute animals in a living state, and many of them 

 greedily attack putrid matter. 



The means employed by molluscous animals to bring 

 the food within the reach of the organs of deglutition, 

 are exceedingly interesting both on account of their va. 

 riety and success. Some are provided with tentacu- 

 la for securing their prey, and conveying it to their 

 mouth, as the cuttle-fish ; others protrude a lengthened 

 proboscis, or an extended lip or tongue, and thus bring 

 their food into the mouth. With many, however, which 

 are fixed to the same spot during the continuance of 

 existence, or only capable of very limited locomotive 

 power, successful efforts are made by the animal to ex- 

 cite currents in the water, (for no permanently fixed 

 animals reside on land,) whereby fresh portions of it 

 are brought in contact with the mouth, and its animal or 

 vegetable contents separated. This action is performed 

 in some cases, as among the bivalves, by the mechanical 

 action of the valves of the shell, aided, in many species, 

 by the syphons, while, in others, it is excited by the 

 motion of articulate feet, as in the bernacles. Where 

 part only of any kind of food is taken into the mouth 

 at once, the lips are possessed of sufficient firmness to 

 cut off the requisite portions, or there are corneous 

 mandibles to perform the office. 



In the mouth there is scarcely any process perform- 

 ed analogous to that of mastication in the higher orders 

 of animals. When the food is in the mouth, or enter- 

 ing into the gullet, it is mixed with saliva, as in the 

 more perfect animals. The salivary glands in which it 

 is secreted, are, in general, of considerable size, divided 

 into lobes, and, in some cases, separated into distinct 

 masses. In many species the existence of a gullet is 

 doubtful, as the food seems to enter the stomach im- 

 mediately; while, in others, there is a portion of the 

 intestinal canal which has some claim to the denomina^. 



tion. 



2 



