ORGANS OF DIGESTION. 385 



and size, and form, not only in the different species, but in 

 the same individual, at different periods of its development, 

 secrete a thick turbid fluid, not unlike the product of ordi- 

 nary muciparous follicles, or those considered as rudiments of 

 salivary glands ; they are connected together by a loose 

 cellular tissue, and numerous plexuses of vessels, and they 

 admit the digested food of the intestine freely into their 

 interior, like the biliary tubuli of many mollusca. The spleen 

 is generally single, small, of various forms, attached to the 

 side of the stomach, as in higher classes, largely supplied 

 with lymphatics, and without perceptible duct ; but in a few, 

 as the sturgeon and the shark, it is divided into detached 

 lobes, as in some of the cetacea ; and in some fishes, as the 

 lamprey, which has neither pancreas nor gall-bladder, nor 

 mesentery, it appears to be wanting, as in the invertebrated 

 classes. The liver attached to a tendinous diaphragm, 

 is of great size, and of an elongated form, placed on 

 the median plane, of a light colour, filled with an oily 

 fluid, soft in texture like the spleen, deeply divided into 

 numerous lobes, with the acini well marked, and the compo- 

 nent tubuli comparatively large, provided with a portal and 

 arterial circulation, and commonly with a large gall-bladder, 

 as in other predaceous vertebrata. It is sometimes placed 

 more to the left than to the right side ; there are generally 

 several long hepatic ducts, terminating separately in the 

 lengthened cystic, several hepato- cystic ducts, entering the 

 fundus of a pyriform gall-bladder, and the common choledo- 

 chus, short and wide, opens along with one or more pancreatic 

 ducts on the anal side of the pyloric valve. The activity of 

 the secretions, and the muscular strength of the alimentary 

 canal, effect a rapid assimilation of food in the short intes- 

 tine of fishes ; and they disgorge by the mouth the shells or 

 other hard parts of their prey, like many predaceous animals 

 of higher and of lower classes. The air-sac, or rudimentary 

 lungs, generally communicates by means of a membranous 

 trachea or ductus pneumaticus, with the intestine, the sto- 

 mach, or the O3sophagus ; but however useful for progressive 

 motion, or the transmission of sounds, it still contributes 

 little to the aeration of their blood. 



The form and extent of the alimentary canal of fishes and the 

 condition of their chylopoietic glands vary as much as their 



PART IV. C C 



