448 



SECRETION. 



with a finely granular matter, intermingled 

 with nucleolo-nucleated bodies, and the an- 



Fig. 312. 



Biliary Organs of Artacus affinis. 



a, crecura of the liver of Cray-fish, with its contained 

 cells ; b, c, d, e, /exhibit the progressive changes 

 of the cells, as they advance from the bottom 

 of the tube. ( After Leidy. ) 



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 caeca 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 Pyc- 

 nogonidcc, in which the biliary apparatus is as 

 much diffused as in the Radiata. In these 

 animals, the stomach sends caecal 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 Molluscous animals, the general 



structure of the liver closely corresponds 

 with that which has just been described in 

 the higher Crustacea. Among the Compound 

 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 Ascidians, 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 folio wing 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 in fig. 313. a. " When one 



Fig. 313. 



Biliary Organs of Helix albolabris. 



a, portion of the liver of the snail, moderately mag- 

 nified, exhibiting the arrangement of the lobules ; 

 b, a biliary csecum from the same liver, highly 

 magnified. 



of the bulbiform caeca (Jig. 313. 6) is examined 

 beneath the microscope, it is found to have a 

 structure differing in no important particulars 

 from that of the cray-fish. The cells at the 

 bottom cf the sac (Jig. 314. a, 1, 2) average 



