FISHES 



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 



FIG. 156. Transverse section 

 of a Fish, diagrammatic. 

 en, Centrum ; cod, coel- 

 ome ; d.a, dorsal aorta ; 

 d.f, dorsal fin ; d.m, dorsal 

 muscles ; d.ms, dorsal 

 mesentery ; f.r, fin ray ; 

 gon, gonad ; int, intestine ; 

 l.v, lateral vein ; msn, 

 mesonephros ; msn.d, 

 mesonephric duct ; n.a, 

 neural arch ; p, parietal 

 layer of the peritoneum ; 

 p', visceral layer ; p.c.v, 

 posterior cardinal vein ; 

 pn.d, Miillerian duct ; r, 

 ventral rib ; r', dorsal rib ; 

 sp.c, spinal cord ; t.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, 1 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 Lepidosteus 2 a ventral mesentery 



p.c.v. 

 msn. d -. 



1 Howes, P.Z.S. 1890, p. 669. 



2 Balfour and Newton Parker, Phil. Trans. 173, 1882, p. 425. 



