The next class of animals, the EPI ZOA, pre sent us with a beautiful series of gradations of development, clearly demonstrating the insepara ble relation that must exist between the nervous and locomotive systems. The Epizoa seem, indeed, to be the osculant group interposed between the Intestinal worms and the lated classes, and exhibit in a permanent con dition the progressively improving external articulated limbs, which are only permitted to attain their full development in higher races of the animal creation. The Epizoa, like the Ccelelmintha, are parasitic in their habits, living, however, upon the external surface, and not in the interior of other animals. They are prin cipally found fixed to the eyes, the skin, the gills, or even the inside of the mouth of fishes, or to the branchial organs of various forms of aquatic animals, from which they suck the materials necessary for their support, and at the same time are freely exposed to the influences of the surrounding medium for the purpose of respiration. In the humblest of these parasites the structure of the body is scarcely superior to that of many Ccelelmintha, stickers and pre hensile organs placed in the vicinity of the mouth being their only means of adhering to the surface upon which they live; but in the Ler wens the first appearance of outward limbs begins to lie perceptible, not as yet recognisable as legs or locomotive agents, but not the less on that account the first rude sproutirms of members that are to be by degrees perfected in more highly privileged genera. Some of these Lerneans, indeed, present most grotesque shapes, and almost exactly resemble the em bryos of Vertebrate animals at the period when the first huddings of limbs begin to project from the sides of the body. This resemblance, indeed, is far more real than it would at first appear, inasmuch as there is a parallelism to be established between the permanent condi tion of the Lernean and the transitory state of the embryo at the corresponding period of its development that is strictly physiolojeal. The condition of the nervous system in them both is precisely similar, exhibiting in both cases the nematoneurose type; the same rudimen tary condition of the muscular system is con sequently equally met with in the embryo and in the I.ernean, but as the nervous system in the former is rapidly advancing to a more exalted state of development, so do the limbs and the muscles appertaining to them improve in the same ratio.
In the higher genera of Errzoa minute ganglia exist in connexion with the nervous filaments, and in such the limbs are of course more exactly formed and begin to sketch out as it were the limbs of Crustaceans and other Articulata.
The study of the muscular system in the extensive class ECHINODERMATA, the last of the nematoneurose division of the animal world, is invested with considerable interest on account of the very different kinds of locomotive appa ratus that successively make their appearance; for as the outward form of these elaborately con structed creatures changes through all the phases represented by the Eocrinite, the Starfishes, the Echinus, the Holothuria, and the worm like Siponculus, the muscles successively assume a different arrangement.
The Encrinite in its outward form might be mistaken for a polyp, the jointed calcareous stern whereby it is fixed to the rock, the body and rays around the mouth, as well as the appendages to the stem, being all in their essen tial structure exactly comparable to those cor tacal polyps, that have an internal jointed cal careous basis of support. The numerous pieces that compose the skeleton of the Encri nite are all invested with a living contractile crust, whereby, in fact, they were secreted, and which forms the bond that connects them together. The living crust that covers the Encrinite can scarcely, indeed, as yet be looked upon as being muscular, so soft and acrite does its composition appear to be; nevertheless in these Echinoderms it seems to be the only moving power employed, and by its slow con tractions bends the arms, or rays, or stem in any given direction.
In the long-armed Starfishes, such as the Co»latule, Gorgonorephali, and Ophiari, the slender and flexible rays around the body are in like manner covered with a living contracting skin, more dense and coriaceous than that of the Encrinite, but still presenting very dubious appearances of muscular fibre, whereby the movements of the rays are effected. The rays themselves constitute the instruments of pro gression, and by their aid these Polyp-like creatures crawl at the bottom of the sea, or by entwining them around the sea-weed that covers the rocks climb in search of food.
In Asterias and kindred forms the exterior of the body is still encrusted with the same contractile covering, and can be bent to a certain extent ; but in these short-armed Star fishes the rays have become so short and devoid of flexibility that they can no longer be useful for the purposes of locomotion : an additional muscular apparatus is therefore now conferred in the extraordinary system of protrusible suckers, that become the chief agents in walk ing, or in seizing prey.
As we advance from the Asterichr we find that the rays at length totally disappear: the body assumes a pentagonal form (Pabnipes), then circular ( Scutella), and at last is enclosed in an ovoid or globular shell, as in Echinus, Cidaris, &c. In these spherical Echinoderms the external soft and living crust that still covers the exterior of the shell presents obvious claims to muscularity, more especially where it passes on to the articulated spines that are attached to the surface of the shell and now become the principal means of progression. The suckers, however, that formed the only locomotive organs in the Asteride still are met with in the Echinidez,—these and the spines constituting an apparatus for locomotion, which, for its complexity, is unparalleled in the ani mal creation. In the Echinidw, moreover, a strangely constructed set of dental organs are developed,and for these likewise special muscles are appointed, for a description of which the reader is referred elsewhere. (See EcnIao DERMATS.) In the Ife/othurider the shell of the Echinus is no longer secreted, and the living integument itself constitutes the whole parietes of the body, which now becomes quite soft and flexible, clearly commencing that transition which is to connect the Echinodermata with the Annulose division of the animal kingdom. The suckers of the last family are, however, still persistent and form the principal means of moving from place to place. The texture of the fleshy skin of the Holothuria is dense and coriaceous, and strong bands of muscular fibre arranged in five divisions pass in a longitudinal direction, from one end of the body to the other, between which circular and transverse fasciculi are dis tinctly perceptible. Imbedded in the muscular walls that enclose the visceral sac of the Ilolo thuridw, delicate nervous filaments are to be de tected passing along the body from end to end, and most probably these are connected together by a circular filament surrounding the oesophagus. Ganglia, if they exist at all, have, from their minute size, hitherto escaped observation ; and, as might he expected from such a condition of the nervous system, the muscular contractions of the fibrous integument, although associated through the medium of the nerves, and con sequently far stronger and mere energetic than in the lower asteroid Echinoderms, are still almost entirely uncontrolled by the influence of volition; nay, so remarkably is this the case, that in most species of Ilolothuria, upon the applica tion of the slightest stimulus to the exterior of the body, or even by simply taking them out of the water in which they live, such violent and general contraction: of the whole integu ment are excited that the intestines and other viscera are forced extensively through the anal orifice, and it is almost impossible for the anatomist to procure a specimen of these crea tures without finding it more or less spoiled from this circumstance.