BREATHING MECHANISM OF THE LAMPREYS. 99 



cult to see the need of a contrivance to prevent the repeated 

 inspiration of the same water. The expired water is thrown out 

 with great force and to a considerable distance like a stream from 

 a hose. It is thoroughly mixed with the adjacent water which 

 by this means as well as by the movements of inspiration is kept 

 constantly agitated. When we remember the rapidity with which 

 gases diffuse through water thus agitated (Hoppe Seyler, 1896) 

 there can be little doubt that the water inspired by a Petromyzon 

 is of practically the same gaseous content as the adjacent water 

 and is unaffected by the expired water. Probably a special con- 

 trivance for preventing the repeated inspiration of the same water 

 is little more needed in Petromyzon than a similar contrivance for 

 preventing the repeated inspiration of the same air in a mammal. 



The current seen issuing from the nostril at each expiration is 

 caused by the compression of the nasal coecum. The caudal end 

 of this ccecum is bounded above by the cephalic end of the noto- 

 chord and below by the first two gill pouches (Fig. I, /"). As 

 the gill sacs shorten with each expiration the ccecum is pressed 

 against the notochord and water is forced from it. With the 

 lengthening of the gill sac the walls of the coecum are again 

 separated and water is drawn into it. This current seems to 

 have no other purpose than to bring the water in contact with 

 the olfactory epithelium. 



B. Respiration u'/icn the Animal is not Attached. The acts 

 of feeding and breathing have now been considered in the normal 

 attached animal, and no use has been found for the velar valves, 

 the velar jaws, the water tube or the valves of the external 

 branchiopore. 



When a gill sac expands there is created a negative pressure 

 within it. To relieve this, water must enter the sac either 

 through the internal branchiopore or through the external 

 branchiopore or through both. When the animal is attached 

 water enters necessarily through the external branchiopore alone. 

 When the animal is not attached the mouth is open and water 

 must enter through the external branchiopore, the valves of which 

 are so constructed that they can offer no resistance to an inflow- 

 ing current, but water may also conceivably enter the mouth, 

 pass the velar valves and enter the gill sacs by way of the water 



