BREATHING MECHANISM OF THE LAMPREYS. 97 



Many factors vary the rate of the respiratory movements, such 

 as the vitality of the individual animal, temperature and the 

 oxygen content of the water. 



The movement when the animal is attached is regular unless 

 foreign particles get into the gills, in which case spasmodic con- 

 tractions of the gills take place in order to expel the irritating 

 particles. These contractions are essentially the same as in regu- 

 lar respiration except that they are longer continued and stronger, 

 so that the external branchiopores are brought closer to the 

 water tube. If many solid particles are suspended in the water 

 the rate of breathing becomes slower and may even stop for a 

 full minute, but if the animal is removed to clean water breathing 

 again becomes normal. 



During both expiration and inspiration the external branchi- 

 opores are wide open and look like so many gaping, round 

 mouths. They do not change their form in either inspiration or 

 expiration. The ectal valves are not taut; with the aid of a lens 

 the ectal valves may be seen napping idly back and forth in the 

 opening (Fig. 10, tf). With each inspiration the ental valves are 

 seen to be swept back into the gill pouch much as swinging 

 doors might be. At each expiration the out-flowing water 

 sweeps the ental valves forward and out through the external 

 gill opening past the loose border of the ectal valve (Fig. 10, r). 

 It is thus clear that in the normal respiration of the attached 

 lamprey the valves of the external branchiopores have no office 

 other than to perhaps aid in directing the out-flowing water 

 backward. 



The movements of respiration and the working of the ental and 

 ectal valves could be seen somewhat more plainly in Iclitliyo- 

 myzon concolor than in L. wilderi. Besides the regular respira- 

 tory movements recorded for L. rvilderi a slight backward and 

 forward motion of the branchial basket was observed. 



From the arrangement of the muscles of the gill sacs and gill 

 pouches it may be inferred that the expulsion of the water is 

 brought about by contraction in all directions of the lumen of 

 the gill sac, while at the same time the long axis of the sac is 

 more shortened than any of its other dimensions. Thus the 

 capacity of the gill sac is reduced and a part of the water is ex- 



