BREATHING MECHANISM OF THE LAMPREYS. 



be pulled at right angles from the surface to which it is attached 

 only by the exertion of considerable force. 



10. It follows from 9 that Lainpctra wildcri attached in a cur- 

 rent of water may retain its hold without nepessarily exerting 

 muscular energy. 



11. In inspiration in the attached Lampetra wilderi, water enters 

 each external branchiopore and the nasal tube, coming gently 

 from all directions, just as it enters the mouth of an empty, sub- 

 merged bottle. It does not enter merely at an angle of 90 to the 

 long axis of the body as stated by Gage for Petromyzon unicolor. 



12. In expiration water leaves each external branchiopore in 

 a backwardly directed stream which forms an angle of 45 with 

 the long axis of the body. At the same time a stream issues 

 from the nasal sac. 



ri 



13. No muscular mechanism has been found to account for 

 the expansion of the gill sacs by means of which inspiration is 

 effected and the inpiration is therefore attributable to the elas- 

 ticity of the cartilaginous branchial basket. 



14. During expiration the gill sacs are compressed and their 

 long axis shortened by the action of the following muscles : a, 

 the internal and external compressors of the gill sac ; b, the 

 superficial compressors of the gill pouch (a muscular pouch 

 which encloses each gill sac and is separated from it only by a 

 large lymph space). 



15. Water is forced out of the nasal opening at each expira- 

 tion by the compression of the nasal ccecum between the noto- 

 chord and the adjacent first and second gill pouches. 



1 6. Water is drawn into the nasal opening at each inspiration 

 by the expansion of the nasal ccecum due to its attachment to 

 the adjacent medial walls of the second and third gill pouches 

 which are separated by the elongation of the axis of these 

 pouches during inspiration. 



17. During both inspiration and expiration the external bran- 

 chiopore has approximately circular form and the valves guard- 

 ing it (ectal and ental valves of Gage) are seen to flap idly in 

 and out of the opening. 



1 8. The ectal valves may, as claimed by Gage, help to direct 

 the inflowing curreVit of water to the central part of the gill sac, 



