20. 



GILLS. 

 Figures 3, 19 and 20. 



Althovigli the gills lia-re been well treated by Mitsu- 

 kuri, ^ ) Kellog^C'') , and ot?aers, some additional facts botli on 

 tlie structrire and function of tJieso organs, seem to make it allow- 

 able to go over the already rather familiar ground. 



Each gill, figure 3, g, consists of a double series 

 of rather broad, flat plates, placed laterally edge to edge, and 

 antero-posteriorly with their flat surfaces opposed. 



This double series of plates is suspended from the body 

 wall by a thin muscular membrane. Anteriorly the gills pass be- 

 neath the oyerha:aging digestive glands, on the under sides of 

 which the plates become gradually smaller and smaller, and final- 

 ly fade away. Ihe extreme anterior ends are continued as 

 ridges. These ridges seen to represent continuations of the 

 suspensory membranes rather than gill plates, and point to the 

 probable origin of the membranes as folds from the body T/all. 



Posteriorly the gills likewise become smaller, and are 

 finally attached to the wall that separates the inhal ent^cuad ex- 

 halent siphon. 



ffith the exception of a few of the plates at the extreme 

 anterior ends of the gills, which are sometimes much distorted 



