98 JEAN DAWSON. 



pelled. The lateral end of the muscular pouch carries inward 

 with it the elastic branchial basket to which it is attached. When 

 the muscles relax, the elasticity of the branchial basket serves to 

 elongate the gill sacs and thus to fill them again with water. 

 The action may be likened to that of the mammalian lung in 

 which the lung is filled b^ muscular action and emptied chiefly by 

 the elasticity of the thoracic walls and lungs. The gill sacs of 

 the lamprey are, on the other hand, filled by the elasticity of the 

 branchial basket and emptied by muscular action. Not all the 

 air in the lungs is changed by a single respiration and not all the 

 water in the gill sacs can be changed at a single respiratory 

 movement. The expulsion of the water at the angle of 45 

 does not retard but rather aids the progressive movement of the 

 fish, while it permits the external branchiopores to have such a 

 form and point in such a direction that they offer the minimum 

 resistance to the movement of the animal through the water. 



Gage's (1893) statement that the expired water leaves the 

 external branchiopore at a very oblique angle with the long axis 

 of the body while the inspired water enters at an angle of 90 

 suggests the possibility that during expiration the major axis 

 of the gill sac forms a very oblique angle with the long axis of 

 the body but that during inspiration this angle becomes a right 

 angle. The following experiment was tried to find out whether 

 there was any change in the obliquity of the gills during respira- 

 tion : A bristle with as large a tip as would enter the external 

 branchiopore was passed through the gill sac into the water tube 

 so that it occupied the major axis of the gill sac. The end of 

 the bristle was left projecting from the external gill opening and 

 formed an angle of 45 with the long axis of the body. This 

 angle remained constant during both inspiration and expiration. 



Gage (1893, p. 469) believes this arrangement by which water 

 leaves the gills at a very oblique angle while the inspired water 

 enters at an angle of 90 is a contrivance to prevent the re- 

 peated respiration of the same water. The water does indeed 

 leave the gill at a rather oblique angle (45), but if the obser- 

 vations here made on the current entering the gill openings be 

 correct, the water flows in from all directions much as it enters 

 the mouth of an empty submerged bottle ; moreover it is dififl- 



