Ixxii Introduction. 



surface of the intestine. In the three more highly organized of the 

 four orders just mentioned, a duodenal segment is distinguishable 

 in the small intestine anteriorly to its valvular portion. This seg- 

 ment is known as the ^ Bursa Entiana' in Elasmobranchii, where it 

 is of considerable size, and marked externally by the entrance of the 

 functional Ijiliary and pancreatic, and the rudimentary omphalo- 

 mesenteric ducts. The rectum always opens anteriorly to the 

 urinary and genital ducts ; except when these tubes open upon its 

 dorsal surface near its termination, so as to constitute a cloaca. 

 The suspensoiy mesenteric laminae often become largely fenestrated, 

 or may disappear altogether in consequence of absorption, in adult 

 Fish. A liver is always present ; it is ordinarily unilobar ; in some 

 Fish it is multilobar; in the Cyprinoids it is trilobed, and interdi- 

 gitates with the convolutions of their intestine, much as the lobes of 

 the liver do in many of the Gasteropoda. There may be several gall 

 ducts, and a gall bladder is very rarely wanting. Secretory coeca, 

 the so-called ^pyloric appendages,' are developed in many Fish 

 upon the commencement of the intestine. They are very variable 

 in number, and ordinarily simple and distinct, though sometimes 

 ramified and bound together more or less closely. A glandular 

 pancreas of smaller size but more compact structure^, coexists some- 

 times with these pyloric appendages. The heart consists of a 

 branchial auricle and ventricle, to the former of which a sinus 

 venosus is superadded, and to the latter, except in Marsipobranckii, 

 an arterial bulb, which breaks up into branches corresponding in 

 number to the gill arches. In the Ganoidei and ElamiohrancUl, 

 the arterial bulb has a layer of transversely striped muscular tissue 

 in its walls, and several rows of valves in its interior; whereby 

 it is enabled, as in Amphibia, to act as an accessory ventricle. 

 The muscular fibre of the arterial bulb of Teleostei is not of the 

 striped variety, and the bulb has only two valves internally. In the 

 Dipnoi a second auricle exists, which receives blood brought back 

 to the heart from the pulmonary air sacs. The systemic aorta is 

 formed, in the embryo, by the confluence of the aortic arches into 

 which the bulb divides ; and, after the development of the gill 

 fringes, for the supply of which these arches resolve themselves 

 into efferent branches, by the confluence of the efferent branchial 

 vems with which those afferent vessels are continuous through the 

 intermediation of the aerating capillary plexuses. In the Bipnoi, 



