26o 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 

 (Pig. 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 {Myxine). In the Gillaroo Trout {Salmo stoviacMcus)f 

 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 {Tinea vulgm-is), 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, 

 Zehrb. d. Eistol. d. ilenschen u. d. Tiered 1857 ; Id. Beitr. zu inikroslc. Anat. u. 

 EniwicM. d. Roclien u. Haie, Leipzig, 1852 ; Id. Anat.-histol. Untersuch. iib. Fische 

 ■u,. Septilien, Berlin, 1853 ; Molin, Sitz. d. Ic. Akad. d. Wiss. zu Wien, v. 1850, 

 p. 416 ; Macallum, Proc. Canadian Inst. N.S. ii. 1884, p. 387 ; Id. Journ. Anat. 

 and Fhys. xx. 1886, p. 604 ; N. Parker, Trans. Roy. Irish Acad. xxx. 1893, p. 109 ; 

 Ayers, Jen. Zeitsch. xviii. 1885, p. 479 ; Edinger, Ardiivf. mikr. Anat. xiii. 1876, 

 p. 651 ; Trinkler, Archivf. mikr. Anat. xxiv. 1884, p. 174. Also Oppel, Lelirb. d. 

 vcrgl. mikrosk. Anat. d. TVirbeltiere, i.-ii. Jena, 1896-97, where nnmerous other 

 references are given. 



^ Owen, op. cit. p. 418. 3 Qwen, I.e. 



