Il6 THE AMERICAN ARBACIA 



When about two months old, five tube feet appear at one side of the 

 body in a sort of pocket, but soon extend out radially; they have 

 suckers at their extremities and are very active, expanding and con- 

 tracting (Photograph 7). Within the next two weeks, 15 petal-like 

 structures appear, three adjacent to each tube foot; these are the primi- 

 tive spines (Photograph 8), The pluteus had already reached its max- 

 imal development, and the arms their maximal length, about i .6 mm. ; 

 the whole animal including arms was over 3 mm,, maximal diameter. 

 A good diagram of this stage is given by Gordon (1929, p. 29). The 

 length of the anal arm at different ages is given in Table 6. A mathe- 

 matical treatment of the data has been given by Glaser (1950), 

 showing that they fit closely with a modified version of Huxley's allo- 

 metric equation. 



TABLE 6 



APPROXIMATE LENGTH OF LONG (aNAl) ARM FROM BASE TO TIP (iN fi) 



The head of the pluteus has remained for some time. The arms now 

 or sometimes before this begin to degenerate, the flesh peels off leaving 

 the bare skeleton (Photograph 6), and they are gradually lost. The 

 metamorphosed animal now consists of the greenish spherical body 

 with 5 tube feet and 15 primitive spines (Photograph 9, 10). The tube 

 feet soon increase in number, the newer ones being more slender 

 (Photograph 11). This is the latest stage obtained in the laboratory; 

 the animal measured about i mm. including spines, and was about 

 3^ months old. The times given for different stages are only approxi- 

 mate as they vary greatly in different lots. 



A very young adult Arbacia was found in July 1952 in a clam shell 



