24 



ed, to be fibrous tissue, but there seems to be no doubt that 

 they are m^^scles. Their contraction cause the gills to sliorten, 

 a coiTimon movement, already described by Kellogg ^'^), and a move- 

 ment necessarily of ser^/ice in their special function. 



The cavities of the plates on the two sides of each gill 

 open ^1 ly into each other, and the cavities of all of the plates 

 are put into communication through the blood space, lbs, ^Afcich 

 is continued upward between the plates as a narrow slit reaching 

 to the upper longitudinal muscle. Above this muscle, at the 

 base of the suspensory membrane, is another blood apace, ubs, also 

 running the length of the gills^t separated from the blood spaces 

 between the muscular walls of the suspensorj?- mejubrane only by oc- 

 casional strands of connective tissue. This space comnuinicates 



,^^ti- est^^-rCtu^ a-^ (11) 



laterally with.each of the gill plates OTavitios . Mitsul-iuri 



supposed that these blood channels^ were connected with some defi- 



nite blood circulation (in ITucula^ 



In Yoldia there seems to be no evidence that the blood 

 follows a very definite course. The continuovis longitudinal 

 spaces seem to be the means of equalizing pressure throughout the 

 gills, by allowing a free movement of the blood to different 

 parts as circijmstances demand. 



ShOTild the blood be confined in any part of the gills 

 when pressure is applied, the walls of the plates would probably 



