MEMBRANES, OVARIES, AND OVIDUCTS OF SALMONOIDS. 1 87 



investing membranes (peritoneal membranes) associated with the digestive and repro- 

 ductive systems. 



Alimentary tract. — In the Salmonidae the alimentary tract forms a loop within 

 the anterior half of the abdominal cavity or coelome, so that three portions are recog- 

 nized: The stomach (fig. in), a thick-walled arm extending backward to the point 

 where it makes a sharp bend and as the pyloric arm (fig. i o), more or less covered by 

 the mass of pyloric appendages or caeca (fig. i />), extending forward to the posterior 

 surface of the liver, where another sharp bend occurs and from which the intestine 

 (fig. I q) extends back to the vent. 



lyiVER. — The liver (fig. 2 /) is relatively massive and fills nearly the whole anterior 

 end of the abdominal cavity, on each side more or less overlying the other anterior 

 viscera. 



Kidneys. — ^The kidneys lie immediately below and in contact with the dorsal surface 

 and extend from the anterior septum or diaphragm (fig. i k) to the region of the vent. 



Pancreas. — The pancreas is an elongated lobulated digestive gland, more or less 

 embedded in fat, lying on the upper surface of the stomach and often more or less upon 

 the upper surface of the intestine posteriorly to the stomach. 



Spleen. — The spleen (fig. i m) is a dark-colored lymphoid or fluid gland of varia- 

 ble size, irregularly a three-surfaced pyramid, situated close behind the posterior curve 

 of the stomach. 



Air bladder. — Immediately below the kidney mass, in contact and approximately 

 coextensive with it, is the air bladder. 



Gonads. — The reproductive glands of the Salmonidae are paired, more or less 

 symmetrical organs, one on each side of the abdominal cavity- 



THE PERITONEUM AND SUPPORTING MEMBRANES OF THE VISCERA. 



The peritoneum is a serous membrane lining the adbominal cavity and sending 

 out various folds which support and more or less attach to each other the visceral 

 organs. Anteriorly, in conjunction with other tissues, it fonns a partition analogous 

 to the diaphragm of higher vertebrates, separating the adbominal cavity from that 

 part of the coelome containing the heart, gill, esophagus, etc. (fig. i k). A fold extend- 

 ing to the digestive organs, infolding and forming suspensory membranes, or filamentous 

 and ligamentous attachments, is called the mesentery (fig. i i- and i). 



Histological structure and embryonic development. — According to Bridge 

 (1904), the peritoneum histologically consists of a stratum of connective tissue, support- 

 ing on its free surface an epithelial stratum (coelomic epithelium). Primarily, the invest- 

 ing peritoneum is continued both dorsally and ventrally into bilaminar suspensory folds, 

 the dorsal and ventral mesenteries, 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 peritoneum lining the whole of the inner surface of the body wall. 

 Embryologically, the two mesenteries owe their formation to the fusion above and 

 below of the mesenteron of the contiguous walls of two laterally and primarily distinct 

 Cffilomic cavities. The dorsal mesentery in tlie adult is occasionally complete, as in the 

 myxinoid Cylostomata and in a few teleosts, but much more frequently is reduced by 

 absorption to anterior and posterior rudiments, or to a series of isolated bands, or even, 



