ACRITA priv. stem), discerno,) a pri mary division of the animal kingdom founded by Virey, and so called by Macleay,11 composed of the lowest classes of the radiate animals of Cuvier, and characterised by an indistinct, dif fused, or molecular condition of the nervous system.
The necessity for a dismemberment of the Radiata of Cuvier, which Rudolphil justly calls a chaotic group, has been felt, and directly or indirectly expressed, by most naturalists and comparative anatomists." It is impossible, in deed, to predicate a community of structure in either the locomotive, excretive, digestive, sensitive, or generative systems, with respect to this division, as it now stands in the " Regne Animal." As in the animal organization the nervous system is that which is subject to the fewest varieties, and as its relative perfection is the surest indication of the relative perfection of the entire animal, the modifications of this system necessarily indicate the highest or pri mary divisions of the animal kingdom, and form their distinguishing characters.
Taking, then, the nervous system as a guide, the radiata of Cuvier will be found to re solve themselves into two natural groups of which the first, composed of the Polyastric In fasoria of Ehrenberg, the Polypi of Cuvier; the Entozoa purenchymatosa, Cuv. or Sterel mintha, and the .Acalephee, differs in the absence or obscure traces of nervous filaments from the second division, including the Echinoderma,' the Entozoa cavitaria or the epi zoa, and the Ehr., in which nervous filaments are always distinctly traceable, either radiating from an oral ring, or distributed, in a parallel longitudinal direction, according to the form of the body.
These different conditions of the nervous system are accompanied with corresponding modifications of the muscular, digestive, and vascular systems, and a negative character, ap plicable to the higher division of Cuvier's Radiata, may be derived from the generative system.
With respect to the muscular system, we find that although all the Acrita possess the loco motive faculty at some period of their exist ence, and many never become fixed, yet that distinct muscular fasciculi are not necessarily developed. In the fresh-water polype, for ex ample, the whole of the homogeneous paren chyma of which it consists is equally con tractile; and even in the medusa, which ranks among the highest of the Acrita, no distinct muscular organs for effecting the contractions of the gelatinous disc have yet been detected.
In the higher division of radiata, on the other hand, which from the filamentous condition of the nervous system may be termed Nemato neura, the muscular system is always distinctly eliminated.
The difference in the condition of the diges tive system between the Acrite and Nemato neurons classes is still more striking : in the former the alimentary canal is excavated in the parenchyma of the body, and is devoid of dis tinct parietes : in the Nematoneura it is pro vided with a proper muscular tunic, and floats in an abdominal cavity.
A corresponding difference is presented by these two divisions of the invertebrate animals, in the condition of the vascular system. Where traces of sanguiferous organs are met with in the Acrita, they are equally with the digestive organ devoid of proper parietes, but consist of reticulate canals in the substance of the body, generally situated near the surface, and in which a cyclosis of the nutrient fluids is observed analogous to that of plants, but not a true circulation. This structure obtains in the Acrita as low down in the scale as the poly gastrica, in which class Ehrenberg has deter mined the existence of a superficial network of vessels containing an opaline fluid. In those genera of sterelmintlia or parenchymatous in testinal worms which manifest traces of the circulating system, the fluids undulate in canals of a similar structure, as is displayed in the planarke, and parasitic trematoda, and also in the echinorhynchi, in some species of which genus the cutaneous canals form a rich net work.* In the acaleplue the condition of the vascular system is equally simple with that of the lowest Acrita, as is exemplified in the mar ginal reticulate canals in the disk of the rhizos toma. In the Nematoneura, on the contrary, those classes which manifest a circulating sys tem distinct from the digestive tube, as the echinoderm and rotifera, possess vessels with proper parietes, distinguishable into arteries and veins.