260 FISHES CHAP. 
muscular coat, (2) the submucosa, and (3) an epithelial stratum 
or mucous membrane, the first two of these layers, with the 
addition of the peritoneum, being derivatives of the inner or 
splanchnic portion of the embryonic mesoblast.’ 
Excluding the oesophagus, where the muscular coat is mainly 
composed of striated fibres, the musculature of the alimentary 
canal usually consists solely of non-striated, spindle-shaped fibres 
disposed in two layers, an external stratum of longitudinally 
arranged fibres, and an inner stratum of circularly disposed fibres 
(Fig. 157), with the addition, in the stomach, of an oblique layer 
between the two. In the oesophagus the reverse arrangement 
may exist, the circular layer being external and the longitudinal 
internal. The muscular coat varies considerably in thickness in 
different regions and in different Fishes, and in the Cyclostomata, 
the Holocephali, some Teleosts, and the Dipnoi may be very 
feebly developed, or even entirely absent, as in the intestine of the 
Hag-Fish (Myzxine). In the Gillaroo Trout (Salmo stomachicus), 
on the contrary, the distal section of the siphonal stomach has its 
musculature unusually thickened, so as to form an incipient 
gizzard for the crushing of the shells of the freshwater Molluscs 
on which the Fish feeds. In some of the Mullets (Mugilidae), 
a true gizzard is developed by the enormous thickening of the 
muscular coat of the caecal stomach, the cavity of which, in 
consequence, is reduced to a mere vertical fissure, and is lined by 
an exceptionally thick, horny epithelium. 
There are a few exceptions to the rule that the muscular 
fibres are of the non-striated variety. Thus in some Teleosts, as in 
the Tench (Zinca vulgaris), striated fibres are continued from the 
oesophagus into the walls of the stomach and intestine, and there 
form an outer longitudinal and an inner circular layer, situated 
externally to the corresponding layers of the non-striated stratum. 
1 For the histology of the alimentary canal and its glands in Fishes, see Leydig, 
Lehrb. d. Histol. d. Menschen u. d. Tiere, 1857 ; Id. Beitr. zu mikrosk. Anat. u. 
Entwickl. d. Rochen u. Haie, Leipzig, 1852; Id. Anat.-histol. Untersuch. ib. Fische 
u. Reptilien, Berlin, 1853; Molin, Sitz. d. k. Akad. d. Wiss. zu Wien, v. 1850, 
p- 416; Macallum, Proc. Canadian Inst. N.S. ii. 1884, p. 387; Id. Journ. Anat. 
and Phys. xx. 1886, p. 604; N. Parker, Trans. Roy. Irish Acad. xxx. 1893, p. 109 ; 
Ayers, Jen. Zeitsch. xviii. 1885, p. 479; Edinger, Archiv f. mikr. Anat. xiii. 1876, 
p. 651; Trinkler, Archiv f. mikr. Anat. xxiv. 1884, p. 174. Also Oppel, Lehrd. d. 
vergl. mikrosk. Anat. d. Wirbeltiere, i.-ii. Jena, 1896-97, where numerous other 
references are given. 
2 Owen, op. cit. p. 418. 3 Owen, J.c. 
