90 



Dr. J. F. Gemmill. 



[Jan. 17, 



still, it gives sufficient purchase to enable urchins of large size to progress by- 

 short steps, while urchins of the smaller and medium sizes can travel over it 

 almost as quickly as over wood or plasticene. At the same time, while the 

 longest steps I observed on glass measured about £ inch, the records on 

 plasticene showed some steps which rather exceeded £ inch. A certain 

 amount of slipping must accordingly have taken place on the glass, and this 

 is borne out by the facts which are noted at the end of next paragraph and 

 also on p. 97. 



Strength of Effort. — Not only are the lantern muscles strong enough to 

 raise and push the urchin intermittently in the manner previously described, 

 but they are able to effect this, even although the shell be loaded with a 

 considerable weight. Progression by short lurches can still take place in 

 specimens of 3 inches diameter under loads of half a pound and under, while 



Fig. 5. — Photograph (natural size) of track left on plasticene by a loaded urchin travelling 

 out of water (p. 90). Movement was from right to left. The heavy dragging 

 of the spines will be noted. Five steps were taken, and then the urchin could 

 advance no further, but was only able to bite at the plasticene, making a large 

 pit, at the bottom of which is seen the white cardboard backing on which the 

 plasticene was spread. 



larger urchins are just able to move along for short distances carrying as 

 much as 15 or 16 ozs. Under these circumstances the impressions left by 

 the teeth on surfaces prepared to receive them are deep and decided, the 

 length of each step is short, and movement is impossible over smooth hard 

 surfaces, on which the teeth and spines can obtain no purchase. 



