244 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



and small intestines may be subdivided, chiefly by differences in their 

 walls. Thus in the small intestine there are recognized in different 

 groups a jejunum, a spiral valve region and an ileum, while the 

 large intestine may furnish a colon, a rectiun and a cloaca, the latter 

 being the part of the canal into which the urogenital organs open. 

 In the cyclostomes but two regions occur, the intestine and the 

 rectum, differentiated externally by the larger 

 size of the latter. In the petromyzonts there 

 is an internal fold of the intestine which pur- 

 sues a slightly spiral course, constituting a 

 spiral valve, a structure which reaches its 

 highest development in the elasmobranchs. 



In the elasmobranchs the intestine is 

 nearly straight, but its differentiation has 

 proceeded farther. At the junction of smaU 

 and large intestine is a dorsal blind sac, the 

 rectal or digital gland. Its function is un- 

 known, but it may correspond to the caeca of 

 the higher groups. In the 'small' intestine 

 is the spiral valve which has two forms, both 

 leading to increase of surface. In most species 

 a fold, carrying blood- and lymph- vessels, 

 arises in a spiral line from the wall of the tube, 

 and its free edge projects into the lumen like 

 a spiral stairway (fig. 257). In a few forms 

 (Carchariidae, Galeocerdo) the hne of origin of 

 the fold is straight and its free margin is coiled 

 hke a roll of paper (fig. 258). In the large in- 

 FiG. 259.— Digestive testine rectum and cloaca are recognized, the 



'jr,L;^Scifor.ter «=!<«<=* ^dng that part which receives the ends 

 H bile duct; g6, gall bladder; of the excretory and reproductive ducts and 



<, hver; /t, large intestine; /"C, • u i.i, j- *• j -4. i • 



pyloric c«ca; si, smaU in- thus IS both digestive and urogenital m 



character. 



testine. 



Ganoids and dipnoi (fig. 256) also have the intestine nearly straight and 

 a spiral valve, least developed in Lepidosteus. In the teleosts the canal 

 may be straight (fig. 259) or may make more or fewer coils, the predaceous 

 species being simplest, while in the mullet (Mugil) there may be thirteen or 

 fourteen turns. In the teleosts the hne between small and large intestine is 

 often marked by an ileo-colic valve and a few species have a cascum or rectal 

 gland. A spiral valve rarely occurs in teleosts and a cloaca is never found. 



