38 THE AMERICAN ARBACIA 



The age of an individual can be told in some cases by the rings of 

 growth on each genital plate, these being marked by an annual deposit 

 of pigment (Deutler, 1926; H. B. Moore, 1935a, 1937; Yonge, 1949, 

 p. 166). Increase in size of the test is brought about both by accretion 

 to the old plates and addition of new ones. 



c. Regeneration 



Though regeneration is not so marked in sea urchins as in starfish, they 

 can regenerate spines, tube feet, pedicellariae and spheridia, and even 

 large fractures of the shell are healed (Mortensen, 1927 a, p. 262; 

 Grasse, 1948, p. 149). Chadwick (1929) reports a C3.se of^ Echinus escu- 

 lentus in the aquarium at Port Erin, which had lost all its spines, fallen 

 to the bottom and seemed dead, but then regenerated its spines. I 

 have observed many Arbacia with apparently regenerated spines, and 

 have observed the process in a few cases ; they regenerate fairly rapidly. 

 A medium-sized animal measured 5.2 cm. with spines, the test without 

 spines 2.8 cm.; the spines removed measured 1.2 cm. Three rows of 

 spines were removed on June 18, 1952. They regenerated at the rate 

 of a little over i mm. a week; in a month they measured 0.5 cm. and in 

 two months (Aug. 17) they were almost completely regenerated. H. 

 W. Jackson (1939), in a similar experiment with A. punctulata obtained 

 similar results; the spines (10-12 mm.) regenerated 5 to 6 mm. in a 

 month. 



For regeneration of spines in Strongylocentrotus, and references in the 

 literature, see E. F. Swan, 1952. 



The regenerative power of the pluteus oi Arbacia is great; an arm 

 is regenerated in a few days (E. B. Harvey, 1949). 



d. Habitat, Burrowing Forms and Collecting 



I. - Rock-Boring Forms 



Arbacia punctulata, like most sea urchins, which are all sea forms, live 

 in aggregations which are often widely separated. They live on rocky 

 or shelly bottoms, or adhere to rocks, but do not make excavations in 

 rocks as do several other species, especially those exposed to rough seas. 

 One of the rock-borers is Echinometra lucunter in Bermuda, where it is 

 known as the "rock" or "reef" urchin (H. L. Clark, 1899; Verrill, 

 1907; E. B. Harvey, 1947). Paracentrotus lividus is found in holes in the 

 rocks in exposed places as observed and pictured long ago by Rondelet 

 (1554). It is found especially on the west coast of France (Cailliaud, 



