NATURAL HISTORY 37 



come from newer beds recently discovered. A similar situation was 

 found by H. B. Moore (1934) for Echinus esculenlus near Port Erin. 

 One bed, at Breakwater, had no small ones, none less than 7 cm. while 

 two other beds, at Brest and Chickens had mostly small ones, under 

 4 cm. 



The best figures for the growth of sea urchins over a period of time 

 are those of Elmhirst (1922) for the large urchin Echinus esculentus and 

 of Bull (1939) for the smaller Psammechinus miliaris, given in the fol- 

 lowing table: 



Diameter of test (without spines) in cms. 

 Echinus esculentus. (Elmhirst, 1922 at Millport, Scotland; Mortensen, 1927 a, p. 299) 



Just 6 mos I yr 2 yrs 3 yrs 4 yrs 5 yrs 6 yrs 7-8 yrs (probably) 



metamorphosed 2 4 4-7 7-9 9-1 1 — — 15-16 



mature 



Psammechinus miliaris. At Cullercoats, Northumberland (Bull, 1939) 

 0,1 0.33 2.0 2.62 2.92 3.03 3.7 3.87 



mature 



Figures for the size oi Psammechinus miliaris in different months and 

 years are given by Lindahl and Runnstrom (1929). The Indian form 

 Salmacis bicolor at Madras was found to measure 4-5 mm. at 3 months, 

 13 mm. at 6 months, and 16 mm. at 12 months (Aiyar, 1935). 



There are no figures published for Arbacia punctulata owing to the 

 difficulty in keeping the animals over winter. The growth during the 

 three month season (June to September) at Woods Hole was found to 

 be only about i mm. even when well fed with Fucus and Cliona 

 (E. B. H.) A group of ten, of approximately the same size, had an 

 average diameter (test) of 22.4 mm. on June 23, 1951 and 23.5 mm. 

 on Sept. 24, an increase of i . i mm. in three months. ' These were kept 

 at Woods Hole over the winter, but owing to lack of running sea water 

 for several days only one survived. It measured 25 mm. on May 29, 

 1952, an increase of 1.5 mm. during the eight winter months. Judging 

 from meager data, a medium sized Arbacia test grows about 3 mm. a 

 year, and continuously through the year. A very young Arbacia, the 

 smallest one yet collected, was 3.6 mm. (test) on July 23, 1952 and 

 5.2 mm. on Sept. 28, a growth of 1.6 mm. in two months. The percentage 

 increase in size is much greater in the small one, as one would expect. 



According to H. B. Moore (1934, 1935 a, 1937), the growing period 

 of Echinus esculentus is limited to the spring months, when spawning 

 also takes place. The duration of life is 4-8 years. 



' Milligan (1916, Zoologist 20 : 399) found that Echinus (Psammechinus) miliaris (27 mm. 

 diameter without spines) increased in the laboratory about i mm. in 5 months. 



