They were chiefly such pulmograda as have their disc bell-shaped that were formerly sup posed to be agastric. It was imagined that alimentary matter being received within the campanulate depression, its orifice was con tracted, and nourishment taken up by im bibition through the walls of the disc. But an attentive examination of Carybdea mar supialis, (Peron,) one of the animals which was believed to be agastric, has satisfied Dr. iVlilne Edwards that a mouth and an internal cavity connected with it do really exist. The great transparency of this animal renders the dis covery of its internal structure a matter of con siderable difficulty, excepting when coloured injections are used. Dr. Edwards found within the funnel-shaped cavity of Carybdea, and, as it were, pendent from its roof, a projection of very delicate tissues, evidently forming tenta cula surrounding a central mouth, and a stomach, from which proceed four long canals leading to the tapering filaments which hang down from the margin of the body of the animal. These canals, Dr. Edwards believes to be analogous to the radiating vessels of rhizostoma. There exists just at the com mencement of each canal, and opening into it, a group of minute cylindrical sacs, which may be regarded as biliary organs.t But in most of the pulmograda these organs are situated on the margin of the disc. Generally, they pre sent the appearance of glands, being distinctly granular in their structure. They are opaque, have a lengthened form, and are lodged in little depressions, and surrounded by cup shaped folds of the external integument. They are connected with the gastric appendages by small tubes.$ In Aurelia phosphorm, (Lam.) (Pelagic, Esch.) which formed the principal subject of Spallanzani's observations on the acalephm, there are four groups of membranous tubes, convoluted, and resembling in structure the intestines of vertebrate animals. Although he did not trace their connexions, Spallanzani appears to have regarded them as truly parts of the alimentary canal. He observed that they exhibit a peristaltic motion, both in the water and in air, which can be increased by the application of stimuli.§ The food of the pulmograda consists of various marine animals—small crabs, and worms. Even large fishes are some times found entangled amongst the arms and tentacules. They are probably killed by the peculiar excretion which covers the surface of these organs, and which produces a stinging effect on man. The long filamentary appen which hang from the margins of the disc in Carybdea and others, are covered with a glutinous matter to which passing objects ad here ; the animal has the power of stretching them out and withdrawing them at pleasure, and of so folding them inwards as to carry to the mouth whatever may be attached to their sides. It would appear that some species are endowed with the power of discriminating the food most suitable to their own nature. Gaede remarks that he has never found fishes in the stomach of medusa capillata, but often worms ; while in that of medusa aurita there are fre quently fishes, rarely worms. In none of the pulmograda have either masticatory or salivary organs been discovered.
The cirrigrada have, in the middle of their lower surface, a large flask-shaped stomach, the mouth which is formed like a sucker. There appears to be a communication between this organ and the numerous tentacula which surround the mouth, through minute canals. The food consists of small animals, such as entomostracous crustacea; the undigested re mains of which are again ejected through the mouth.
IV. Circulation.—No distinct circulating system has hitherto been discovered in the acalephw. • But perhaps the peculiar apparatus of radiating vessels connected with the gastric cavities in the pulmograda, and the aquiferous canals of the ciliograda, which seem to per form nearly the same functions as the vascular system of higher animals, may be conveniently and properly considered under this head. .
In the physograda, Eschscholtz saw what he considered as the rudiments of a circulation ; namely, distinct vessels arising from the roots of the tentacula, and ramifying on the in ternal surface of the air-bladders ; hut it does not appear that he traced these further, or that he saw the movements of a fluid within them.
The vessels in the ciliograda, within which a fluid is seen to move, are situated chiefly beneath the cilia-bearing arches. This fluid is supposed by most modern anatomists to be merely water ; but by some it is regarded as a peculiar fluid, the product of the animal's digestive powers. If it be water only, the canals in which it moves must be considered as being analogous to those of the aquiferous system of other classes of invertebrate animals, which has been so fully illustrated by the re searches of Delle Chiaje,* and which is pre sumed to be subservient , to the respiratory function. The vessels in question arise in Beriie from a vascular circle which surrounds the intestine near the anus. They are eight in number, and one runs beneath each cilia-bear ing arch, from one extremity of the body to the other. They then terminate in another annular vessel, which surrounds the mouth. In their course they give off numerous branches. From the oral circle of vascular structure arise two large vessels, which run along the walls of the gastric cavity, and ap pear to unite with the other circle at the anal extremity. These last Eschscholtz regarded as veins, and the eight external vessels as arteries. He supposed that the veins, passing along the walls of the stomach, absorbed the nutri ment, and then carried the circulating fluid to the cilia for aeration. In the course of his observations on the Beriie ovatus, Dr. Fleming* distinctly saw a fluid moving " backwards and forwards" in the external vessels ; and he states that " while the animal was active, there were numerous small spaces in the different vessels where the contained fluid circulated in eddies." Dr. Fleming failed to detect any structure in the vessels which could produce these partial motions. In cestum naiadis, Eschscholtz thought that he saw the system of vessels more dis tinctly than in any other of the acalephm. He thus described it : " From the base of each of the two tentacules, a vessel takes its rise, and goes towards the bottom of the stomach. Here the two vessels unite, and form a little vascular circle around the water-canal (intestine). From the upper margin of this circle, four straight vessels arise, which go towards the two rows of cilia-bearing organs placed on the dorsal sur face. Under these they run, two in one di rection, and two in the other. At either extremity of the body, these unite with certain vessels running superficially along the sides, and which complete the circulation by entering the first set of vessels just before they begin to run beneath the ciliated organs. All these vessels are simple canals, of the same diameter throughout, without any visible branches. They contain a colourless watery fluid, in which mi nute yellowish globules are seen to move. In the vessels which arise from the bases of the tentacules, the globules mount upwards ; they assume a rotatory motion in the vascular circle; and, in the four dorsal vessels, they seem to move, some in one direction, others in the other. It is probable that what appears to the eye as one vessel, is, in reality, composed of two vessels, running parallel and close together."t Seeing that the radiating vessels which arise from the gastric cavities of the pulmograda seem to carry out the nourishing material to all parts of the body, and that they are, in some species at least, connected with other vessels which form a complete circle, we are disposed to class them under this head along with the vascular structures already described. The exact analogies of their functions, however, have not yet, we conceive, been distinctly made out.