92 



Dr. J. F. Gemmill. 



[Jan. 17, 



general result of this line of experiment and of the plasticene records is to 

 show that urchins travel very slowly, and never further than an inch or two, 

 if their lanterns are not in use. 



Urchins not infrequently cast off all but the smallest of their spines, 

 and renewal and re-growth to normal size are probably slow. At any rate, 

 among equal-sized urchins, one finds extreme differences as regards the 

 number and size of the spines. There are, however, no corresponding 

 differences in the rates at which the urchins travel out of water. Indeed, the 

 average results, as worked out from a large number of instances in which the 

 relative lengths of the spines were noted, showed that the advantage, as 

 regards speed and distance travelled, lay rather with the short and medium- 

 spined urchins than with the long-spined ones, in cases where the lantern 

 of Aristotle had been in use. 



Rhythm. — The time required for the various movements involved in a 

 single lurch is fairly uniform for the same urchin if travelling undisturbed, 

 but shows some irregularity when changes of direction are adopted. The 

 rate of swing is only slightly accelerated by stimulation, but becomes retarded 

 as vitality is lost. The average frequency varies slightly among healthy 

 specimens of similar size, but shows marked variation according to differences 

 in size, the smaller urchins having higher frequencies. The following data 

 may be quoted from examples actually observed : — 



Equatorial shell- 

 diameter of urchin. 



Time in seconds between six 

 successive " steps." 



Average length of 



™" 



H 

 2i 

 3— Si 

 4 

 5 



15, 16, 14, 15, 15, 16 

 20, 23, 22, 22, 21, 23 

 25, 27, 18, 18, 26, 27 ■ 

 36, 37, 50, 50, 36, 36 

 45, 44, 46, 46, 44, 45") 

 50, 52, 65, 44, 51, 49/ 



^of an inch. 



Ta — nr 



Length of " Step." — As indicated in the last column the length of step 

 varies according to size, the smaller urchins naturally taking the shorter 

 steps. At the same time, after an equatorial shell diameter of 2>\ inches has 

 been reached, there is practically no further increase in the length of the- 

 steps. Urchins of 1\ — 3£ inches proved the best for travelling. The larger 

 urchins are more sluggish than those of medium size, and their weight is 

 probably greater in proportion to the size of the lantern and the strength of 

 its muscles. 



Change of Direction. — It is well known that urchins will change their line 

 of progression to a direction away from a part irritated. Out of water, a 



