In the higher Crustacea we find a condition of biliary structure much more closely allied to that of Mollusca than to that of Insects ; the liver being a pair of massive lobulated bodies, each of them made up by the aggre gation of numerous cmcal follicles, from every one of which passes off a narrow duct, to join a trunk that is common to all the vesicles on one side. " In structure," says Dr. Leidy (loc. cit.), " the *cmca resemble the tubes of insects, being coinposed of a sac of basement membrane, within which, originating from the inner surface, are numerous secreting cells (fig. 312. a). The cells are more or less polygonal in form, from mutual pressure. At the bottom of the cmca the cells are small, with an average diameter of -02 millim., and contain a finely granular matter of yellowish hue, with a granular nucleus, and a trans parent nucleolus. As we proceed from the bottom upwards, the cells (f, e, d, c, b) are found to increase in size, and to obtain a gra dual addition of oil globules, until beyond the middle of the tube, where they are found filled with oil, so as to have the appearance of ordinary fat cells, and have a diameter avera ging .06 minim. From this arrangement of the cells, when a cmcum is viewed beneath the microscope, its lower half appears filled with a finely granular matter, intermingled with nucleolo-nucleated bodies, and the an structure of the liver closely corresponds vvith that iv hich has just been described in the higher Crustacea. Among the Conzpound Tunicata, however, to which the Bryozoa are so nearly related that many naturalists asso ciate them together in one group, the struc ture of the liver is the same as that of Bower bankia ; the hepatic follicles being isolated from each other, and lodged in the walls of the stomach, into the cavity of which they pour their secretion by separate orifices. In the Solitary ilscidians, the hepatic follicles are more developed, and cluster round the ex terior of the stomach, so as to give it a shaggy appearance, very much as in the earthworm. In the Conchifera, the liver presents itself as a distinct organ, composed of numerous lobules ; each of these is made up of a cluster of tubes, terminating at one extremity in flask-shaped follicles, whilst at the other they coalesce into a few larger trunks, which discharge themselves into the digestive cavity. The follicles are filled with cells containing the biliary secretion. The structure is nearly the same in the Gasteropoda ; the ducts of the several lobules coalescing, so as to form two main trunks, by which the secretion is poured into the duodenum. The following is Dr. Leidy's account of the minute structure of the liver of the snail ; a portion of which, moderately en larged, and showing the arrangement of the lobules, is shown infig. 313. a. " When one terior half with a mass of fat cells, the nucleus hardly visible, from the property of oil ren dering organic tissues more or less trans parent. The central cavity of the ceeca is filled with fat globules, and a finely granular matter corresponding to that in the interior of the cells." In some of the lower forms of Crustacea, the liver is reduced to the simple condition which it presents in insects ; and there is one very curious group, that of Pyr nogonitIce, in which the biliary apparatus is as much diffused as in the Radiata. In these animals, the stomach sends cmcal prolonga tions into the legs, and these extend nearly to their terminal claws. The walls, both of the central stomach and of its tubular exten sions, are studded with brownish-yellow cells; but beyond this there is no rudiment of any organ for the secretion of bile.
In the Molluseous animals, the general of the bulbiform ceeca (fig. 313. b) is examined beneath the microscope, it is found to have a structure differing in no itnportant particulars from that of the cray-fish. The cells at the bottom of the sac (fig. 314. a, I, 2) average .02 millim. in diameter ; those towards the other extremity about .01, millim. Some of stomach gives ofF on either side a number of branches, which usually redivide, and then the fully ripe cells (b, 1, 2) are filled with innumerable minute globules of oil, hardly distinguishable from the granular matter ; others (c, 1, 2, 3) with globules of a larger size ; some are found with frorn one to ten or more large, deep yellow, oil globules in the centre ; a few (f ) with a hard or crystallised mass of fat in the centre ; and many (d) are distended with oil. By pressing the cells (g) between two plates of glass, the contents will be squeezed out, and the structure will be seen as follows :— the vesicular transparent, amorphous cell-wall, finely granular matter, fat globules, and a granular nucleus (11), mea suring about .01 millim. and containing a hard transparent nucleolus. A few of the cells contain two nuclei. The blood-vessels, con sisting of arteries and veins, form a rete around the bulbiform cmca, but do not appear to come in immediate contact with the secreting cells " (loc. cit.). The general plan of struc ture of the liver of the Cephalopoda is essen tially the same ; the hepatic ducts and follicles being clustered as in a raceme, and the follicles being crowded with biliary cells. In the Loligo, these follicles are described by Dr. Williams (op. cit.), as being themselves sac culated, by duplications of their membrane ; and some of the biliary cells appear as if pro ducing a new generation within themselves.
A very remarkable departure from the general type is presented by certain of the NudIbranchiate Mollusca, of which Eolis may be taken as the type. In these animals, the give ofF smaller tubes, which are continued into the branchial papillm that cover the dor sal surface (fig. 315.). " The prolongations of the branches that enter the papillm undergo a considerable enlargement and change of form ; and from the variety and brilliancy of their colouring are the chief attraction of these elegant little animals. The simplest form of this peculiar organ is met with in Eolis concinna, where it is a mere dilated tube, having its walls slightly waved, and the inner surface sprinkled with darkish granules. In E. Farrani (fig. 316. E) it still retains a con siderable simplicity of structure, but becomes decidedly sacculated. The complexity is much increased in E. olivacea, in which it is produced into puckered follicles or sacculi ; but in E. papillose (fig. 316. A) it appears to attain its highest development. The central canal is there somewhat tortuous, and gives off on all sides variously sized, irregular, blind sacs, which are crowded with little compound follicles. The whole of the inner surface is lined with a thickish layer of irregular vesicles or globules, filled with numerous granules. These last, when submitted to a hip,,h magni f)ing power, are seen to be of various sizes, transparent, rounded, and nucleated. The whole of the internal surface of the gland is covered with vibratile cilia. These compound glands are evidently biliary organs, diffused throughout the several papillw, and supplying the place of a compact liver which is wanting in the body of the,se animals. The stomach the limits of each ; they appear to differ in different species. In E. papillosa, the central canal is evidently a continuation of the sto mach, and the plicated internal membrane is not only continuous throughout it, but also passes into the lateral branches, which thus appear to form part of the same organ. On the ether hand, we find in some species coloured granules, similar to those of the papillm, partially lining the ramifications, as in E. gracilis and others ; while in E. despecta, the central canal, all the ramifications, and the glands of the papillm, are coloured and gra nulated alike, implying a greater diff'usion of the biliary function. The food, after being partially digested in the stomachal pouch, is driven in detached portions through the alimentary system, by the alternate contrac tions of the pouch and great trunks leading from it ; these contractions are only of a nature to produce an oscillatory motion, which serves to promote that intimate mixture of the alimentary matters with the hepatic and other secretions, necessary to the process of digestion.* The intimate structure of the liver of Ver tebrated animals is much more difficult of elucidation, and can scarcely be said to be yet satisfactorily determined. The organ presents more and more, as we ascend the series, a solid parenchymatous texture, which strikingly contrasts with its loosely lobulated racemose aspect, even in the highest Invertebrata. There is not the least difficulty in demon strating that this parenchyma is composed of cells, which correspond in the nature of their contents, and, therefore, in their functional character, with those contained within the hepatic follicles of the Invertehrata ; but the point of obscurity is the relation of these cells to the biliary ducts, the arrangement of whose ultimate ramifications has been rather a matter of surmise and inference, than of actual obser vation. It is very interesting to find, however, that in the lowest known Vertebrate the liver exists under the same rudimental and diffused type, as that which it exhibits in the lower Arti culata. In the Amphioxus, or lancelet, the only vestige of a distinct hepatic organ is a large cmcum prolonged from the stomach, which is lined with greenish-yellow cells. But it is pointed out by MiiIler, that the intestinal canal itself has a layer of similar cells in its walls, so that the organ would seem to have the same diffused condition as that which it presents in the earth-worm. In all other fishes, however, the liver is a well-defined conglomerate gland, even the Myxinoids presenting a liver nearly as fully developed as that of the higher fishes, so that there is not here any such complete gradation as we usu ally meet elsewhere. Dr. T. Williams states that he has succeeded in tracing the ducts to their ultimate terminations in the liver of the Sole (Solea vulgaris), and the Flounder (Pla tessa ,flexus); and he describes them as rami fying like those of the Mollusca, and as ending in tubular cmca, without vesicular expansions. Within these cmca are found the hepatic cells, which usually, as in the Invertebrata, contain a large quantity of fat.* There is a remarkable diminution in the proportion of the adipose contents of the hepatic cells, and an increase in the granular constituents, in the class of Reptiles ; and in Birds there is an almost total absence of adipose particles. The ultimate distribution of the bile-ducts, and their relation to the parench3 ma, seem to be the same as in the Mammalia.