AND DEVELOPMENT OF THE LIVER. 
in 
Leech {Hirudo medicinalis), and the Sandworm {Arenicola Phcatorvm) : among these 
we shall find some instructive and remarkable differences, the existence of which 
seems to indicate clearly that we must not rely too much on finding exact analogy of 
structure in any organ, even in the individuals of the same class. In the Earthworm, 
a thin yellow stratum is found applied over a great part of the outer surface of the 
intestine ; it adheres intimately to it, and seems to be moulded on the convexities of 
the sacculi, not dipping deeply into the furrows by which they are separated. I have 
already described this yellow layer as consisting of dark-yellow masses, the majority 
of which cannot be seen to have an enveloping cell-membrane, while in others it is 
clearly perceptible though of extreme tenuity ; the masses, with or without this en- 
velope, are often seen elongated into a conical form, the apex directed towards the 
intestine, to which it often adheres pretty firmly; from this circumstance I formerly 
conceived that the biliary secretion was discharged by the bursting of the cells into the 
cavity of the intestine, each cell representing for a short space an attached follicle, and 
it is not impossible that this may be the case to some extent. But there is another mode 
in which the bile may come to exert its action on the chyle (if such indeed be necessary), 
viz, by the latter percolating the coats of the intestine, and consequently the layer of 
hepatic substance, before it is absorbed by the ramifications of the deep abdomino- 
dorsal vessel. This latter opinion seems to be strongly supported by the unques- 
tionable condition of another part of the intestinal apparatus, known by the name of 
Typhlosole : this blind tube, which is beautifully plicated on its surface, extends along 
the dorsal aspect of the intestine from the gizzard nearly as far as the vent. In struc- 
ture it consists of a strong homogeneous membrane, covered by a layer of ciliary 
epithelium, continuous with that which may be often seen on the rest of the intestinal 
surface, while internally it is lined by a thick stratum of biliary cells, almost precisely 
similar to those on the exterior of the intestine, and forming with them one con- 
tinuous system. Now the typhlosole thus seems to be an inflection of the intestinal 
wall for the purpose of straining off the chyle from the coarse mass of the ingesta; 
and as it is certain that the biliary matter is not discharged from the cells which line 
it into the cavity of the intestine, it seems also probable, that at other parts of the 
intestinal surface the process is the same, the chyle transuding through them as it 
does through the membrane of the typhlosole. 
In the Leech, as Prof. Owen has described it, the hepatic apparatus appears as a 
brown tissue, extending along the alimentary canal between the nervous cords and 
the mucous glands, and also upon the dorsal aspect of the anterior part of the cavity. 
It is composed, he states, of a congeries of elongated, convoluted and irregularly con- 
stricted follicles, which are united in groups by the confluence of their ducts into a single 
slender excretory tube : that this tissue, so well described, is truly the hepatic organ, 
cannot for a moment be doubted by any who have examined it with the microscope, 
the follicles being filled with minute spherules of a deep yellow colour, resembling 
precisely those whieh crowd the biliary cells in the Earthworm. Regarding it then 
