But in every group of animals, as we ap proach the most highly organized members of that group, we find the characters of a more exalted type of organization beginning to ma nifest themselves, and thus in the Artiniode, which are obviously osculent between the Acrite Polyps and animals possessing a true muscular system, a fibrous arrangement of the contracting portions of the body becomes very distinctly recognisable, and a nervous filament may be displayed under favourable circum stances passing round the oral extremity of the creature, and thus closely approximating the nematoneurose type of structure. The infuso rial animalcules ( Polygostrire, Eh ren .) seem, as Far as relates to their muscular system at least, to be strictly acrite animals, but such is their extreme minuteness, that much uncertainty still necessarily exists concerning their intimate organization. Their locomotive apparatus most frequently consists of fringes of vibratile cilia variously disposed, the movements of which are most probably dependent upon the exist ence of a peculiar vital tissue distinct from muscle. In many species, e. g. the Protects (Ameba detests), the contractions of the body are extensile, so that even the outward form of the animalcule is perpetually changing, and some, the Vortirell6e,are attached to highly imitable pedicles of exquisite tenuity, that may be straightened or suddenly thrown into close spiral coils by some inherent power, the nature of which is as yet quite incompre hensible. In some, as for instance Chilodon unrinatos, moveable hooks are found to be ap pended to the surface of the animalcule, and some ( Nassuk) are provided with a peculiar dental apparatus, consisting of a minute cylin der of horny filaments; nevertheless no appear ance of muscular or nervous fibre has as yet been detected even in the largest and most conspicuous species.
The ACALEPLIX next claim our notice as members of the Acrite division of the animal creation, and in every point of their economy they strictly conform to the general characters belonging to this type of organization. (See Acura.) Their bodies are soft, pellucid, and gelatinous, without any trace of muscular fibre being perceptible in their composition ; their digestive canals are excavated in the paren chyma of the body, not contained in any abdo minal cavity, and the canals through which nutriment is conveyed to different parts of the sys tem are entirely devoid of proper external coats; neither, believe, do nerves exist in any of the class, although, as we are well aware, two eminent observers have entertained a con trary opinion. Professor Grant,* in his account of the anatomy of Cydippe pileus, describes a double ring of nervous fibre as surrounding one end of the alimentary canal of that beau tiful little Aca!eph, and has even figured ganglia distributed at intervals upon these circular cords from which secondary nerves are de scribed as emanating. Such a circumstance as the existence of nerves and ganglia in an ani mal confessedly acrite, and presenting no traces of that type of structure which, in all other cases, invariably accompanies so elevated a condition of the nervous system, from its very singularity was well calculated to attract the notice of the physiologist, and we are ourselves quite satisfied Oat the distinguished professor has been led into error upon this point, most probably from having mistaken the circular canals, described by Delle Chiaje and others, as surrounding the oral extremity of the Beroes, and which are indeed frequently filled with an opaline fluid, fur nervous fibre.t
Professor Ehrenberg has more recently an nounced that he had detected a nervous fila ment encircling the margin of the disc in one of thepulmonigrade Acaleplia, and connecting the red spots, which he is pleased, as we think without sufficient reason, to call eyes, with each ether. Such a nervous system would, at least, be anomalous; and, notwithstanding the justly high reputation of the eminent professor of Berlin. we cannot but think that the interests of physiology require us to pause before we assent to the views of Professor Ehrenberg,as re lates to the nature of the filament in question.
In the Sierehnintha or parenchymatous En tozoa of Cuvier, the same acrite condition, both of the nervous and muscular systems, is still observable in the lower forms belonging to this group of internal parasites, which being, for the mostpart, in their natural situations either en closed in cells or closely imbedded in the sub stance of the viscera of other animals, could not be expected to have any power of locomo tion conferred upon them ; their bodies are, therefore, in the simplest species (such as the Ilydatids, Cysticerci) mere membranous bags of homogeneous texture, and without a trace of fibre in their composition : their powers of moving are proportionately feeble, and are limited in fact to slight contractions, which are but indistinctly perceptible on the application of stimuli to the surface of the living animal.
In the Tape-worms ( Teenite) the presence either of nerve or muscle is equally impercep tible, and the whole structure strictly conform able to the Acrite type. As, however, we mount higher in the scale of organization among these parasites, we again find how nearly succeeding types of structure are made to approximate, and even to a certain extent to become blended, as it were, with each other. In the Flukes ( Distoma) and kindred genera, and in many of the Acanthocephalous Sterel mintha, although their structure is evidently parenchymatous, the skin, without presenting any decided appearance of muscular fibre, be comes more coriaceous and contractile, and at the same time nervous filaments become dubi ously perceptible : a transition is obviously in progress, and thus we are gradually introduced to another and a more elevated series of animals.