1912.] The Locomotor Function of the Lantern in Echinus. 107 



and fall of pressure within the whole internal cavity of the urchin to cause 

 swelling and shrinkage of the gills when the walls of the latter are relaxed. 



I have not yet had the opportunity of repeating Uexkull's experiments 

 with due care on Echinus esculentus, hut certain considerations make it very 

 doubtful whether similar results would be got. To begin with, in the 

 Mediterranean species, according to Uexkull, the gill-walls are flaccid, while 

 the walls of the dental sacs are markedly contractile. In Echinus, on the 

 contrary, the gill-walls have contractility and tonicity, while the dental sacs 

 are very thin and are only weakly contractile. Lastly, to judge from 

 Uexkull's figures, the sub-radial sacculations of the lantern coelom (which 

 are the portions of this cavity that could be most readily compressed or 

 released from pressure) reach a greater size in Sphferechinus than in our 

 Clyde species. On the whole, therefore, from the structural point of view, the 

 lantern coelom in Echinus does not seem well adapted to serve as an actively 

 pulsating chamber for the rhythmic filling and emptying of the gills. 



Effect on Water-vascular System. — The internal pressure being lessened with 

 every powerful protrusion of the teeth, any elastic-walled internal cavity 

 which communicates with the exterior will have a tendency to expand. 

 Should the communication be valved against reflux, turgescence of the cavity 

 will be produced or maintained. It seems to me that we have here a 

 mechanism which may materially assist the ciliation of the stone canal, &c, 

 in keeping up within the ampullae that degree of fullness on which perfect 

 action of the sucker feet must depend. The point is one which would be 

 difficult to verify by experiment, but it is at least in harmony with the fact 

 that no rigidly shelled Echinoderm without a lantern makes effective use of 

 its podia in locomotion. 



Summary. 



I. Locomotion out of Water. — When travelling actively (p. 93) the urchin 

 raises itself from time to time on the tips of its teeth in preparation for a 

 forward " step " (p. 85). The step is then accomplished by means of (a) strong- 

 pushing or poling on the part of the lantern (p. 87) ; (h) similar but usually 

 weaker action on the part of the spines (pp. 87, 91) ; (c) the influence of 

 gravity acting at a certain stage (p. 87). After each " step " the lantern 

 is retracted and swings forward, so that the teeth come into position 

 for initiating a new •' step " (p. 86). Plasticene surfaces may be used with 

 advantage for recording the marks of teeth and spines (pp. 89, 90). The 

 length of each step ranges from ^ toj inch, according to size and activity of 

 specimen, and the time required from 15 to 55 seconds (pp. 92, 93). The 

 longest distances travelled are from 12 to 16 inches in 20 minutes, but hardly 



