104 



Dr. J. F. Gemmill. 



[Jan. 17, 



coelomic lining a thin membrane with fine fibrils resembling those of Echinid 

 muscle. 



We may take it then as certain that, in Echinus esculentus, the gill walls are 

 contractile. 



Further, under normal circumstances, the gill- walls maintain a degree of 

 tonicity sufficient to prevent them from responding readily to moderate rises 

 of internal pressure. One may press on the teeth and oral membrane in a 

 healthy urchin without affecting the gills. Less force similarly applied in 

 the case of an urchin treated as under (e) causes the gills to swell out at 

 once. 



Under ordinary circumstances accordingly, in a healthy Echinus esculentus, 

 it seems most probable that the gill circulation depends — (1) chiefly on the 

 ciliation of the coelomic lining ; (2) in a minor degree on contraction and 

 relaxation of the musculature of the gill-wall, occurring without rhythm and 

 probably by reflex action ; (3) also in a very minor degree on other slowly 

 acting causes, e.g. variations in the total quantity of fluid within the lantern 

 coelom. 



Forced Respiration. — What one might call " forced respiration " can, how- 

 ever, occur. This is evidenced by swelling and shrinkage of the gills, 

 corresponding with that rhythmic functional activity of the lantern whose 

 application to locomotion it was the first object of this paper to bring out. 



Forced respiration of this kind can only take place effectively if the 

 muscular walls of the gills be in a state of relaxation. Experimentally, 

 exposure to air affords a good means of inducing this condition. The 

 exposure should be done, as far as possible, without leakage of coelomic fluid 

 and consequent disturbance of internal pressure conditions. Keeping the 

 urchin inverted for 10 or 15 minutes within a vessel with just enough sea- 

 water to leave the gills uncovered will be found to give the result desired. 

 Subsequent complete immersion, followed, if necessary, by mechanical 

 stimulation, in order to induce or increase the rhythmic lantern movements, 

 allows the effect of these movements on the now flaccid gills to be watched. 



Shrinkage of the gills takes place during protrusion of the lantern. It is 

 best marked towards the extreme of this phase and is followed by turgescence 

 which, occurring with some suddenness at the commencement of retraction, 

 persists throughout the retraction phase, and is even continued for a brief 

 period into the commencement of the succeeding protrusion. The filling and 

 emptying of the gills is quite regular, and affects all of them alike. The 

 rationale of the process seems to be as follows: — The lantern and the oral 

 membrane act as a diaphragm which rises and falls. With each descent of the 

 diaphragm (i.e. during protrusion) the total space within the shell tends to be 



