THE ELASMOBRANCHS 6$ 



This description can happily be supplemented by ref- 

 erence to what may be called the rectal 'salt' gland of 

 the dogfish (analogous to the nasal 'salt' of the marine 

 birds and reptiles described in Chapter XI) . All the elas- 

 mobranchs, apparently, possess a glandular appendage 

 to the gut, located in the dorsal mesentery and draining 

 into the posterior gut behind the spiral valve. 



The function of this gland remained unknown until 

 the summer of 1959, when Dr. J. Wendell Burger, work- 

 ing at the Mount Desert Island Biological Laboratory 

 in Salisbury Cove, Maine, demonstrated that in the spiny 

 dogfish, Sqimlus acanthias, the gland secretes a color- 

 less, neutral sodiimi chloride solution. This secretion is 

 isosmotic with the plasma but contains nearly twice the 

 sodium chloride concentration of the latter (ca. 500 and 

 250 miUimols per hter, respectively). It contains little 

 calciimi, magnesium, sulfate or bicarbonate, and the 

 urea concentration is only one-twentieth that of the 

 plasma. (In its isosmotic nature, the rectal gland secre- 

 tion differs from that of the nasal gland of the marine 

 birds and reptiles, which has nearly twice the osmotic 

 pressure of the plasma.) The maximal rate of secretion 

 in untreated dogfish under experimental conditions is 

 about 1.3 cc. per kg. per hour, a figure approaching or 

 exceeding the simultaneous urine flow. 



This rectal 'salt' gland affords a mechanism by which 

 the dogfish can dispose of excess sodium chloride after 

 the ingestion of sea water— the magnesium, calcium, and 

 sulfate, being poorly absorbed from the intestinal tract, 

 are presumably largely evacuated with the feces, and 

 excess potassium may be excreted by the kidneys or gills. 

 The gland appears to serve only as a protective device, 

 like the nasal gland in the marine birds and reptiles; the 

 present evidence does not indicate that the marine elas- 

 mobranchs habitually drink sea water, as do the marine 

 teleosts. Added to the urea-retention habitus, it guaran- 

 tees the animal a continuous supply of water for the 

 formation of an osmotically dilute urine. 



