258 FISHES CHAP. 
The whole length of the alimentary canal from the oeso- 
phagus to the rectum is invested externally by the visceral 
layer of the peritoneum (Fig. 156), which histologically consists 
of a stratum of connective tissue, supporting on its free sur- 
face an epithelial stratum (coelomic epithelium). Primarily, 
the investing peritoneum is continued both dorsally and 
ventrally into bilaminar suspensory folds, the dorsal and ventral 
mesenteries (d.ms, v.ms), which extend to the mid-dorsal or mid- 
ventral line of the abdominal cavity. The two layers then 
separate and become continuous with the parietal layer of the 
peritoneum lining the whole of the inner surface of the 
Fia. 156.—Transverse section 
of a Fish, diagrammatic. 
cn, Centrum ; coel, coel- 
ome; d.a, dorsal *aorta ; 
d.f, dorsal fin ; d.m, dorsal 
muscles; d.ms, dorsal 
mesentery ; 7.7, fin ray ; 
gon, gonad ; int, intestine ; 
Z.v, lateral vein; msn, 
mesonephros ;_ msvn.d, 
mesonephric duct; 2.a, 
neural arch; p, parietal 
layer of the peritoneum ; 
p’, visceral layer; p.c.v, 
posterior cardinal vein ; 
pn.d, Miillerian duct ; 7, 
ventral rib ; 7’, dorsal rib ; 
sp.c, spinal cord ; ¢.p, trans- 
verse process ; v.m, ventral muscles ; v.ms, ventral mesentery. (Modified, after Parker 
and Haswell.) 
body-wall. Embryologically, the two mesenteries owe their 
formation to the fusion above and below the mesenteron of the 
contiguous walls of two laterally situated and primitively distinct 
coelomic cavities. The dorsal mesentery in the adult is occa- 
sionally complete, as in the Myxinoid Cyclostomata and in 
the Elasmobranch Hypnos subnigrum,' and also in some Dipnoi 
and in a few Teleosts, but much more frequently it is reduced 
by absorption to anterior and posterior remnants, or to a series 
of isolated bands, or even, as in the Lamprey (Petromyzon), to 
a few filaments accompanying the intestinal blood-vessels. The 
ventral mesentery, on the contrary, is rarely present, and if 
present is never complete. In ZLepidosteus” a ventral mesentery 
1 Howes, P.Z.S. 1890, p. 669. 
? Balfour and Newton Parker, Phil. Trans. 173, 1882, p. 425. 
