130 ROBERT TRACY JACKSON ON 



eleventh plate of Fig-. 114 has one spine boss, the eleventh plate in Fig. 115 has in addi- 

 tion several secondary bosses, and in Fig. 116 the eleventli plate has yet more spine 

 bosses, showing a still further development or specialization in the growth of plates which 

 are actually or at least very nearly the equivalent of one another. 



In this specimen of Strongylocentrotus, PI. 24, fig, 116, the primary, secondary, and 

 tertiary spine bosses are represented, but still smaller bosses or miliaries are omitted as an 

 unnecessary and confusing amount of detail. In Fig. 116a, however, all tlie spine Ijosses 

 are represented which occur in the fifteenth plate of the left-hand column of Fig. 116. 



In the growth of this sea-urchin it is interesting to note that the increase in size is 

 attained by the increase in the size of plates i-ather than by the increase in the number 

 of plates. The smallest specimen, Fig. 114, measures 7 mm. in diameter and has 11 plates 

 in a column, whereas the adult measures 52 mm. and has only 22 plates in a column, 

 or twice as many as it had when about one seventh of the size attained. 



Arbacia punctulata. In Arhacia lyunctuJatn Gray, a similar series of facts may be 

 observed and is especially clear on account of the relative simplicity of the interambula- 

 cral plates. A small individual, 23 mm. in diameter (PI. 24, fig. IIU), has eleven 

 plates in each column. The eleventh plate of the left column is very young and has not 

 yet acquii-ed a spine boss ; tliis is a common character of very young plates at this dorsal 

 area in sea-urchins and is similar to the condition seen at the introduction of i^lates in 

 embryonic sea-urchins (Loven, '02, p. 11, Fig. 1). The tenth plate of this specimen has 

 a single spine boss. Passing ventrally it is seen that plates 9, 8, 7 also have a single 

 spine boss, but progressively larger. The sixth plate and others ventrally as far as the 

 second plate all have two spine bosses. In an adult specimen measuring 45 mm. in diam- 

 eter (PI. 24, fig. Ill), there are fourteen plates in each column. The fourteenth plate 

 of the left cohunn is small and has no spine boss, as in the eleventh plate of Fig. 110. 

 The thirteenth ])late has developed one spine boss and in this character is comparable to 

 the tenth plate of Fig. 110. Passing ventrally it is seen tliat the tenth plate has two 

 spine bosses, whereas the tenth plate of Fig. 110 has only one spine boss. The eighth, 

 seventh, and sixth plates have each three spine bosses, whereas the similar plates in 

 Fig. 110 have either one boss or, in the case of the sixth plate, two bosses. The fifth, 

 fourth, and third plates of Fig. Ill have four bosses, showing a progressive development 

 to this point passing ventrally. The second and first plates are simpler, with fewer spine 

 bosses, retaining at this area something of their youthful simplicity of structure. 



In Arhacia j^unctulatn (PI. 24, fig. Ill) it is seen that the genital plate has three 

 pores, an abnormal feature. This occurrence of accessory pores is common in this 

 species. In 244 specimens examined from Wood's Holl, Mass., 3.6 had more than one 

 pore in one or more of the genital plates. The specimen figured has three pores in eacli 



