102 



Dr. J. F. Gemini]!. 



[Jan. 17, 



boring habits elsewhere do not exercise these, where there are no tides, as in 

 the Mediterranean.* 



Gill Respiration. — In the tanks, urchins that are apparently health) 7 some- 

 times have their gills prominent and well filled, while an hour or two later 

 the gills may be by no means conspicuous. Changes of this kind do not 

 seem to be rhythmic in character. Probably they depend on such factors as 

 variations in the total quantity of fluid within the lantern ccelom. A 

 necessary condition is relaxation of the muscular tissue of the gill- walls (see 

 below, p. 103). The oral membrane also will often be found to be flaccid 

 when the gills are largest. But, so far as this paper is concerned, the 

 important question as regards respiration is whether the rhythmic movements 

 of the lantern have any influence in causing flux and reflux of the fluid 

 contained in the gills. This question arises alike in connection with the 

 powerful lantern movements previously described, and with the minor ones 

 now to be noted. Under water, urchins which are otherwise motionless may 

 for hours at a time show slow rhythmic movements of the lantern. These 

 movements correspond with the locomotor ones if one supposes the latter to 

 be greatly reduced. There is slight protrusion and retraction, with opening 

 and closing of the teeth accompanied by lateral swing. The rate corresponds 

 exactly with that in actual locomotion. I have had the opportunity 

 repeatedly of watching the movements in question in specimens which had 

 been left undisturbed for several days in glass-sided tanks provided with a 

 somewhat feeble circulation. On one occasion at 11 a.m. the .positions of 

 several urchins showing the movements were carefully marked, and at 3 p.m. 

 it was found that the urchins had not shifted their position, though the 

 movements of the lantern were still going on. 



Careful watching through the glass failed to reveal any corresponding 

 rhythmic swelling or contraction of the gills. The same thing holds good 

 regarding the major rhythmic movements of the lantern which subserve 

 locomotion. There is, under ordinary circumstances, no obvious increase or 

 diminution of the size of the gills during protrusion and retraction of the 

 teeth. If such variations do occur they are too slight to be distinguishable 

 amid the secondary movements imparted to individual portions of the gills 

 by the spines, tube feet, and pedicellariai which are in constant activity all 

 around. 



Ordinary Respiration. — In considering how renewal of the coelomic fluid 

 inside the gills of Echinus is effected one must not forget two facts, (1) the 



* Fischer, loc.cit., and de Serres, Marc, "Sur Taction perforante de l'Echinus lividus," 

 ' Compt. Rend.,' 1857, vol. 44, p. 72 ; and " Note sur l'Echinus lividus de l'Ocean, 

 considere comme une espece perforante," ibid., 1856, vol. 43, pp. 405 — 6. 



