CRUSTACEA and INSECTS have properly no brain, but only a nervous cord full of knots or ganglia, called the spinal marrow, though improperly, as these ani mals have no spine, and their knotty cord neither re sembles the spinal marrow of other animals, nor is it al ways situated along the back. The nervous system of MoLLusca and WORMS nearly resembles that of insects.
With respect to the organs of the external senses we may first remark, that all the MAMMALIA, CETACEA, BIRDS, REPTILES, and FISHES, appear to possess those of all the five senses, though not in an equal degree of perfection; that the senses of hearing and smelling are found in a great variety of animals, though we cannot always distinguish appropriate organs that are destined to the exercise of these senses; that feeling is perhaps the most generally diffused of all the senses, and that sight is wanting in a great many animals. As in the former part of this article, we shall begin with the or gans of feeling.
The skin in most of the AIAMMALIA is so thickly covered with hair, wool, scales, shells or prickles, that its sensibility to external objects must be greatly di minished, though we know that in several species, as the horse, the ox, and especially the elephant, it is acutely sensible. Some of these animals, as the apes, and other quadrumana, seem to possess in the ultimate divisions of their extremities, organs of touch scarcely inferior to the human figures ; and we know that the trunk of the elephant answers to that animal all the purposes of the human hand. The lengthened snout of the ta pir, and the projecting upper lip of the rhinoceros, probably perform a similar office. Several of the MANMALIA, like the cat, have whiskers which they em ploy in feeling the dimensions of narrow holes through which they have occasion to pass. The tongue in many mammalia seems also to be employed as an organ of touch.
The beaks of BIRDS probably supply to them the want of a more delicate feeling organ. We know that in many species, especially geese and ducks, the beak is abundantly supplied with nerves, and is covered with a very delicate membrane ; and there is every reason to believe that these birds, in dabbling among the mud for their food, are directed in the search by the delicacy of this organ.
It has not been ascertained by anatomists whether REPTILES have any appropriate organs of touch ; but probably most of those with naked skins, as the frog and lizard, employ their skin for this purpose, and to some the tongue appears to perform the same office. The skin of serpents is supposed to be unusually sen sible, and is considered by many as their only feeling organ.
Many FISHES possess a very acute feeling in their belly and their lips, and some of them are furnished with worm-like tentacula, which they half conceal in the sand, and by means of which they receive notice of the ap proach of their prey.
There seems little doubt that in CRUSTAC EA and IN SECTS, the antenna are very delicate organs of touch ; and most naturalists, by giving the name of palpi, or feelers, to be neighbouring prominences, have, whether justly or not we cannot determine, assigned to them the same office.
Many of the MOLLUSC A have either horns or tentacula, which appear to serve the purpose either of feeling or of seizing their prey. The same is the case with the ten tacula of ZOOPHYTES and polypes. Whether WoRMs, properly so called, have any other organs of touch than the delicate skin in which they are inveloped, is uncer tain.
There is less certainty with respect to the compa rative anatomy of the organs of taste, than that of any other of the external senses. We know that most clas ses of animals possess a tongue, but there seems great reason to doubt whether, in a great variety of them, this organ possesses the faculty of tasting. In some of the MAMMALIA, the nervous Izapillx of the tongue are remarkably large and prominent ; and from this circum stance, some physiologists conceive that these animals possess a greater delicacy of taste than man.