440 



THE AMERICAN NATURALIST 



[Vol. LIV 



the majority of the dead tests, which may be collected in quan- 

 tity, are about 10 cm. in transverse diameter. From these esti- 

 mates of age in Mellita, the described growth-changes in the 

 form of the body begin to become obvious during the third year 

 of an individual's life; in some specimens they do not occur 

 at all. 



During the middle years of its life, Mellita has a considerable 

 capacity of withstanding injuries (cf. Fig. 6) and of repairing 

 damage done to the periphery of its body. I have already noted 

 the fact that injured and regenerating individuals are found to 

 have been damaged at the posterior end only; the wound illus- 



12 



Fig. 5. Possible growth curve of M. sexies-perforata. 



trated in Fig. 6 is unusually far forward. It was previously 

 suggested (Crozier, 1919) that the posterior incidence of in- 

 juries resulted from the circumstance that the posterior end was 

 more freely exposed than the rest of the body. Observations in 

 aquaria and on sand-beaches have shown this explanation to be 

 probably correct. Burrowing takes place with the anterior end 

 in advance, the edge of the posterior inter-ambulacral area fre- 

 quently remaining exposed long after the rest of the creature 

 has been concealed beneath the sand. The experimental tests 

 were made with active, healthy, specimens exhibiting no green 

 areas upon their surface (cf. Crozier, 1918). During burrow- 

 ing, the body of the sea-plate is moreover tilted, anterior end 

 down, at an angle of 10°-20° with the surface of the sand; so 



