The NEMATONEVRA, as is obvious from every part of their economy, are gifted with higher attributes, and permitted to enjoy a more extended intercourse with external ob jects than any creatures comprehended under the preceding division. They are no longer rooted to one spot or imprisoned in enclosed cavities, but, on the contrary, are for the most part erratic in their habits, and in many of them the locomotive system is so efficiently con structed that their movements, exhibiting con siderable activity and energy, argue the posses sion of distinct and precisely arranged muscles, and display such combination and consentane ous action of different parts of the body co operating to produce a given result, that the existence of an intercommunication throughout the system by means of nerves might readily be predicated, even had not anatomy revealed to us that such animals actually possess a ner vous apparatus. It would seem indeed to be clearly indicated by the physiological relations that exist between the two systems, that the possession of muscular fibre arranged in dis tinct fasciculi involves, as a matter of course, the co-existence of nervous threads, whereby the actions of distinct and distant muscles may be associated for the attainment of a common object ; and accordingly we find that these two important additions to the animal economy make their appearance simultaneously. No large ganglionic masses are as yet developed of sufficient importance to be regarded as consti tuting a common sensorium, to which the per ceptions derived from external senses must be referred, and from whence mandates of volition can be supposed to emanate. Senses, there fore, that is, localized and special senses, can not as yet be given ; the traces of individuality are but feebly recognizable ; the vital powers are still, to a great extent, diffused throughout the different tissues of the body, and not col lected and concentrated, as in animals pos sessed of brains, that is, of centralized and dominant aggregations of neurine; and, as a consequence of this important circumstance, some of the most striking characters common to the ZOOPIIYTES still linger in this division of creation ; the radiated form is yet extensively met with, multiplication by mechanical divi sion of the body is still, to a certain extent, possible, and severed portions of the body are found to be reproduced by growth from the mutilated part.
The CCELELMINTITA or cavitary intestinal worms, living in the interior of other animals, differ in so many points from the Acrite Entozoa, that Cuvier, although in the Rive Animal he was content to group thin together in the same class, was obliged to separate .hem into two distinct orders, calling the STERELMINTII A " intestiaaux parenchymateax," while the more highly organized are designated " intestinuax cuvitaires." The Caelelmintha, in fact, are or ganized in accordance with quite a different type of structure, as must be at once evident upon the slightest comparison between them. The digestive apparatus is now no longer com posed of tubes excavated out of the general mass of the body, and presenting no outlet for the escape of egesta, but a distinct alimentary canal now makes its appearance, suspended in a capacious abdominal cavity, wherein, more over, are lodged the male and female organs of generation, which in the Ccelelmintha are gene rally found in different individuals. The pa
rietes of the body are in these worms obviously muscular, and are composed of contractile fibres arranged in superposed strata and affect ing different directions. Towards the exterior of the body they are disposed longitudinally, but the inner layers assume a circular or spiral arrangement ; such a disposition providing for the extension or shortening or lateral inflexions of the worm, and enabling the animals so con structed to move about with facility in the cavi ties wherein they reside.
Nevertheless, at this, its first appearance in the animal series, muscular fibre has not as yet attained to the perfection of structure that it offers in the higher classes. The fibres are as yet indistinct, soft, and gelatinous ; they ap pear to be deficient in fibrin ; neither do they, when examined with the microscope, present the transverse striae that are so characteristic of the muscular tissue in a more advanced condi tion. The little fascicles are, moreover, ex tremely short, and run but a little distance before they disappear, and are succeeded by others. Their whole appearance, in fact, is that of muscle in a rudimentary condition, and very accurately resembles the nascent muscular tissue when it first becomes apparent in the embryo of the vertebrate animal. The nervous system accompanying this condition of the muscles is extremely simple ; a delicate ring surrounds the commencement of the oesopha gus, by which, perhaps, the muscles of the mouth are associated during the act of imbibing nourishment, and prolonged from this ring are two long and thread-like nerves, one running along the dorsal and the other along the ventral aspect of the body, and passing quite from one extremity to the other, but without any percep tible ganglionic enlargements in their course.* Having, therefore, no brains or central masses to which perceptions could be referred, localized organs of sense are likewise wanting in all the CCELELM I MIA. The BRVOZOA which, while naturalists were ignorant of their more compli cated organization, were, until a recent period, confounded with the Polyps, are, from their entire structure, very justly entitled to a much higher position in the scale of animals, and un doubtedly belong to the Nematoneurose type. These little beings inhabit transparent cells of very elaborate and delicate construction, from the mouth of which they protrude the anterior portion of their bodies when in search of food. Although from their general appearance the Bryozoa might easily be, and in fact have been until recently, erroneously regarded as Polyps, the differences between the two classes are ex ceedingly striking and important. The Bryo zoa, instead of having the tentacula that sur round the oral aperture quite simple and fila mentary, as the Polyps have, are furnished with ciliated tentacula, and from the rapid ciliary movement which is incessantly going on, while the arms are expanded, strong cur rents are formed in the surrounding water, all of which impinging upon the oral orifice bring to the mouth such nutritive materials as are necessary for the support of the creature.