264 FISHES CHAP. 



stomach, and — excluding the Bursa Entiana where a number of 

 oblique folds are present — of the intestine also, is, on the con- 

 trary, perfectly smooth. 



In addition to transverse and longitudinal folds the mucous 

 membrane of the various sections of the alimentary canal is often 

 developed into outgrowths which are more or less linear.^ In 

 the oesophagus these may be papilliform, as in Box and Caesio ; 

 obtuse in Acij)enser, hard and almost spine-like in species of 

 Bhombus ; or in the form of pyramidal retroverted processes 

 with jagged or fringed edges, as in the Spiny Dog -Fish 

 {Acanthias vulgaris). In the Basking Shark (Selache) similar 

 processes are present, which, near the stomach, become unusually 

 long and branched, so that the entrance to that cavity is 

 surrounded by a series of backwardly-directed arborescent tufts. 

 Peculiar papillose or tag-like processes of the mucous membrane 

 are frequently present on the spiral valve of Elasmobranchs, in 

 the intestine of such Teleosts as Balistes, Mugil and some 

 Pleuronectidae, and also in the rectum of Rhomhus maximus. 



Of all the outgrowths from the mucous membrane of the 

 alimentary canal the so-called " spiral valve " of the Cyclostomata, 

 Elasmobranchs, Holocephali, Chondrostei, Crossopterygii, Amiidae, 

 Lepidosteidae and Dipnoi is the most characteristic. The first 

 appearance of this structure was probably in the form of a straight 

 longitudinal fold or ridge projecting into the cavity of the 

 intestine, similar, perliaps, to the typhlosole of many Inverte- 

 brata. This primitive condition is not retained in any existing 

 Fishes, although it may be closely approached in the larval 

 Cyclostome (Ammocoetes), and is perhaps also indicated in the 

 straight anterior portion of the spiral valve of Polypterus. 

 Absent altogether in the Myxinoids, the valve is represented in 

 its simplest condition, as in certain other Cyclostomata (e.g. 

 Petromyzon), by a ridge of mucous membrane which commences 

 anteriorly on the dorsal side, and, after describing a partial 

 spiral as it passes backwards, terminates posteriorly on the 

 ventral side, the width of the valve not exceeding half the 

 diameter of the intestine. This simple type of valve is repeated 

 in embryo Elasmobranchs, but in the adults of these Fishes the 

 valve becomes much more complicated, and exhibits a wide range 

 of structural variation. The increased complexity of the valve 

 ^ Owen, op. cit. p. 415. 



