108 The Locomotor Function of the Lantern in Echinus. 



50 per cent, of the specimens went more than 4 inches in that time (p. 94). 

 Active progression hy the lantern alone, is possible in small and medium- 

 sized urchins (p. 91). Progression by the spines alone is very limited 

 indeed (p. 91). An urchin can travel with the help of its lantern even when 

 loaded to the extent of half a pound or more (p. 90). There is usually some 

 rotation as well as progression, but the two are not associated as cause and 

 effect (p. 94). Eotation is caused by (a) screwing action on the part of the 

 lantern and of the spines, and (Jb) defect, inactivity, or unfavourable position 

 of a group of spines (p. 95). An analysis is given of the lines or curves of 

 progression in relation to rotation (p. 96). Other points to which attention 

 is directed are — Muscles involved (p. 87), strength of effort (p. 90), change 

 of direction (p. 92), inversion (p. 93), equatorial section (p. 93), recording 

 surfaces other than plasticene (p. 91), the inertia and momentum of the 

 rhythmic action (p. 94). 



II. Locomotion under Water. — Here the lantern is not needed for ordinary 

 locomotion, particularly over more or less horizontal surfaces (p. 97). There 

 are, however, various circumstances, normal and experimental, in which it is 

 employed with effect, as, for example, when the urchins are loaded (p. 97), 

 or travelling up a slope on certain surfaces (p. 98), or only partially 

 immersed (p. 99), or mounting rapidly up a vertical surface (p. 97). 



III. The locomotor action of the lantern in urchins is a particular mani- 

 festation of a rhythmic functional activity which can also subserve feeding 

 (no doubt the chief function, p. 100). boring (p. 101), and " forced respiration " 

 (p. 104). In addition, it possibly aids the swallowing of food (p. 105), the 

 evacuation of faeces (Uexkull) (p. 105), and the maintenance of physiological 

 turgescence in various internal cavities (p. 107). The gill-walls contain con- 

 tractile elements (p. 103) which maintain tonicity (p. 104). Ordinary gill 

 circulation depends chiefly on the ciliation of the coelomic lining (p. 103). 

 " Forced respiration " occurs in connection with the rhythmic lantern move- 

 ments, when the muscular tonicity of the gill-walls is relaxed from any cause 

 Tilling of the gills then takes place during retraction of the lantern and 

 emptying during protrusion (pp. 104, 106). Experiment shows that during 

 protrusion there is a slight lowering of internal pressure within the shell, 

 affecting both the lantern ccelom and the peri-visceral coelom, while during 

 retraction a slight corresponding rise occurs (p. 105). UexktiU's experiments 

 on Sphcerechinus granulans are noticed (p. 106). 



The illustrations include photographs of tracks left on plasticene by 

 urchins travelling actively out of water (fig. 4), partly immersed in water 

 (fig. 8), completely immersed and going uphill (fig. 7), out of water and 

 carrying a load (fig. 5), completely immersed and carrying a load (fig. 6). 



