106 



Dr. J. F. Gemmill. 



[Jan. 17, 



The question now arises, does the gill wall under physiological circum- 

 stances relax sufficiently to facilitate rhythmic filling and emptying ? If water 

 is thoroughly boiled to drive off all dissolved air and then cooled, and if 

 urchins, especially those of small or moderate size, be brought into it, they 

 will climb vigorously up to the surface, exhibiting in many cases rhythmic 

 locomotor movements of the lantern and having the gills well expanded. 

 Not in all, but in about a third of such instances, one can observe some 

 degree of swelling and shrinkage of the gills accompanying retraction and 

 protrusion of the teeth. 



As regards " forced respiration," then, I have shown that in the rhythmic 

 activity of the lantern we have a function capable of emptying and filling 

 the gills when the muscular walls of the latter are relaxed, and that in one 

 particular state of the surrounding water the required degree of relaxation 

 may occur. 



Uexkilll's Experiments. — An interesting and suggestive set of experiments 

 on the factors which control filling and emptying of the gills in the Medi- 

 terranean species, Sphcerechimts granulans, has been made by J. von Uexkull 

 (loc. cit.). He was able (by mechanical stimulation of the nerve ring in an 

 opened urchin which had previously been immersed in sea-water saturated 

 with C0 2 ) to induce rhythmic retraction and protrusion of the lantern. 

 These movements were accompanied respectively by filling and emptying of 

 the gills due to rise and fall of the pressure within the lantern coelom. He 

 states that rise of pressure is caused by compression of the coelom and 

 its sacculations, resulting from contraction of the mouth membrane, the 

 depressor muscles of the radii, and the walls of the dental sacs. Lowering 

 of pressure, on the other hand, appeared to be caused by contraction of the 

 so-called compass sheet of muscle connecting the five forked radii. In single 

 instances, he states that he was able to observe respiratory movements going 

 on while the lantern as a whole remained at rest. Though he mentions the 

 swinging of the lantern to one side or another, he does not recognise its 

 ambulatory functions. He dismisses with less than due consideration the 

 observation by Romanes and Ewart referred to above (p. 84), and states 

 that in the normal medium — sea-water — an urchin never uses its lantern for 

 progression. He does not ascribe to protrusion or retraction of the lantern, 

 in themselves, any influence on the filling and emptying of the gills. 



The chief difference between the kind of respiration induced by Uexkull 

 in Sphaerechinus, and the " forced respiration " of Echinus, is that, in the 

 former, the lantern coelom seems to act by itself as a pulsating chamber to 

 the gills. In contrast I have to emphasise the fact that in Echinus simple 

 protrusion and retraction of the lantern are associated with sufficient rise 



