560 



COMPARATIVE ANATOMY. 



reduced to almost nothing in 



a spiral fold, which is greatly developed in the Selachii, and divides 

 the greater part of the mid-gut into a number of more or less 

 closely applied coils (Fig. 31 G, G vs). In Carcharias this fold has 

 the form of a rolled-up sheet of paper. This spiral valve is 

 retained in the Ganoidei : it 

 Lepidosteus. It is not present in the Teleostei 



An enlargement may be observed at the commencement of the 

 mid-gut of the Selachii ; in the Sturiones there is a large, and 

 externally much diverticulated glandular organ at the same point ; 

 it is divided internally into a number of spaces corresponding to the 

 diverticula. In Lepidosteus the various portions are more sharply 

 separated from one another, and have the appearance of groups of 

 short csecal tubes which beset the pyloric portion of the mid-gut, 

 and, as in many Teleostei, form the appendices pyloricas (Fig. 31 G, 

 A B ap). They beset a certain portion of the mid-gut and vary in 



oe 



Fig. 31G. Enteric canal of Fishes. A Of Salmo salvelinus. B Of TrachinnS 



radiatus. C Of Squatina vulgaris, oe CEsophagus. v Stomach, dp End of 



the air-duct, p Pylorus, op Appendices pyloricas. d Ductus choledochus. vs Spiral 



valve, i Mid-gut. c Hind-gut, ts Its appendage. 



number and size. They sometimes open separately into the gut, 

 sometimes are united into larger trunks, and. give rise to branched 

 structures. They are most numerous in the Gadidas and Scom- 

 beroi'da?. In many Fishes the different creca are held together by 

 connective tissue and united at a common efferent duct, in which 

 case they have the appearance of a compact gland (Scoruberoi'das), 

 while their affinity to the gland in the Sturgeon is implied by the 

 frequent union of their orifices. 



In many Teleostei the mid-gut is much longer than the tract 

 of the coelom which is given up to it, and it is then arranged in 

 coils (Fig. 31G, B i), or in several ascending and descending loops. 

 This implies an adaptation to the cavity of the ccelom, whilst the 

 elongation of the tract, which is always derived from a straight 



