• NI • • 11 The middie pair hears a small palp on its lateral margin. In some species, a small tongue has been found. All these parts bear a close re semblance to the same organs in some of the Crustacea. The cesophagus is short; its lining membrane is somewhat horny, stiff enough permanently to distend the whole canal ; be fore entering the stomach, its diameter is con siderably enlarged. It receives the ducts of two salivary glands. The stomach (c,fig. 341) is capacious ; externally, it presents an irre gular mamillated surface, studded with nu merous small prominences closely set, which are the outer surfaces of hepatic cells, formed in a layer of glandular tissue that closely in vests the walls of the stomach. These cells communicate directly with its general cavity (a,-fig. 342). There is no other organ that can be regarded as a liver.* Two ccecal appen dages, also saccu lated internally, and embossed outwardly, are attached to the stomach.
The intestine is wide, nearly without convolutions, and ta pering towards the anus (d,e, fig. 341).
In the Lepads the stomach is situated in that part of the visceral mass near est to the peduncle ; from which point the intestine runs on the dorsal aspect of the body, and terminates in the anus just at the base of the articulated tubular process. It is slightly dilated near the anus. The walls of the in testine are perfectly smooth and free from folds and duplications. The number of their tunics cannot be satisactorily determined. M. St. Ange has described a singular piece of struc ture which he has found within the intestinal canal of certain Anatifze (c, c, fig. 342). It is a kind of second intestine, which floats within the cavity of the one just described. It is nearly equal in length to the outer canal. Its upper extremity is expanded, funnel-shaped, with edges cut into fringed processes like the mouths of the Fallopian tube in vertebrate animals. These processes are lodged in the cells of the walls of the stomach, and furnish the only means of attachment to the outer walls with which the organ is provided. It thence tapers towards the anal extremity, where it is pointed and closed. Its walls are very thin and delicate. It is generally filled with alimentary matter, which must pass from its cavity by a kind of rumination, so as to enter the stomach a second time.
Circulation.—The sancruiferous system of the Cirripeds has not yet been fully investigated. Only the vessels of the arms, and a central canal, situated on the dorsal aspect of the body, have been discovered. Poli asserted that he saw a heart pulsating a little above the anus : but it does not appear that any other observer has made the same remark. Burmeister has
searched, in vain, for a heart, in the large Coro nula diadema. The vessels of the arms can be distinctly seen through the transparent integu ments of the ciliated processes ; there are, in each process, two vessels, one of which runs very superficially between the two rows of hairs. ( Fig. 343.) Cuvier regarded the anterior canal of the peduncle in Anatifa as the nourishina vessel of that organ.
Respiration.—The principal organs concern ed in respiration are, in the Lepads, certain tapering filamentary processes attached to the sides of the anterior part of the body, which are regarded as the branchke (d, g, fig. 340): in most of the Balanids, they assume the form of two leaf-like membranes with fringed mar gins, and are attached to the inner surfitce of the mantle. Professor Burmeister describes the gills of Coronula diadema as broad mem branous expansions, of a semicircular form, attached to the sides of the visceral mass by a narrow pedicle. They are composed of two tunics arranged in deep and narrow transverse plaits. The number of the branchize in the Lepads varies from four to sixteen. They are composed of soft cellular tissue, and have a smooth surface.
The arms (h, h, fig. 340), which constitute so large a portion of the general mass of all the Cirripeds, and which form their most distinc tive feature, must be regarded as subservient chiefly to the function of respiration; although, by producing currents in the water, which bring food within reach of the jaws, they minis ter also to the digestive function. In all the known species, both of Lepads and Balanids, these arms are twelve in.number, six on either side, arranged symmetrically. Each arm is composed of a short flesby peduncle, having three articulations, and two horny articulated processes, compressed laterally, of equal length, ciliated on their internal surfaces, and coiled up in a spiral of one turn. On their internal surface there is a coating of a black pigment in spots. Each joint is provided with a double row of hairs of different lengths. (Fig. 343.) factorily tbe structure of the secreting apparatus by which the shells of the Cirripeds are formed. In the Lepads, the organs must be imbedded in the ligamentous membrane by which the valves are united : and in the Balanids, they are arranged in six rows along the outer surface of the mantle, and around the base; but, as in acephalous mollusca, they are too small to ad mit of their structure being particularly exa mined. The external surface of the mantle in the Balanids has also the power of secreting calcareous matter, vvith which to increase the thickness of the shell.