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 ow and Caesio ; 
obtuse in Acipenser, hard and almost spine-like in species of 
Rhombus; 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 Salistes, Mugil and some 
Pleuronectidae, and also in the rectum of Rhombus maximus. 
Of all the outgrowths from the mucous membrane of the 
alimentary canal the so-called “ spiral valve” of the Cyclostomata, 
Elasmobranchs, Holocephah, 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, perhaps, 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 
1 Owen, op. cit. p. 415. 
