OCULAR AND GENITAL PLATES. 



107 



smooth and 10 has the primary tubercle. In the adult (text-fig. 94), plates 14 to 16 are smooth 

 and 13 has the primary tubercle. This is in accordance with what I showed (Jackson, 1899) 

 as localized stages in development in Strongylocentrotus and Arbacia, that the young last 

 added plates are simple, and that differential characters are progressively added as the plates 

 grow older and are pushed ventrally by newly added plates on their dorsal border. 



It would be extremely interesting to study a large series of young developing Cenirechinus 

 setosus. I succeeded in getting 110 specimens from 8 to 40 mm. in diameter from the West 

 Indies, Bermuda, and the Pacific. Of this series, as shown in the table (p. 155), 23% have 

 all oculars exsert, 5 % have I insert, 9 % I and V, and 55 % have the adult typical character 

 of I, V, IV insert. Progressive variants in this immature series are infrequent, as would be 

 expected; 5% have I, V, IV, II insert, and 3% have all oculars insert. With later growth 

 unquestionably more oculars would have come in in some of the specimens, so as to give the 

 usual adult percentage of progressive variants. There are two aberrant variants, one with 

 oculars V, IV, and one with I, V, IV, III insert. 



W { O V 



95 



Text-figs. 93-95. — Ocular plate arrangement in Cenirechinus setosus (Leske). 



93. Bermuda. Diani. S5 mm. R. T. J. Coll., 686. X 2.5. Oculars I, V, IV in.sert, the species character. 



94. Florida. Diam. 70 mm. R. T. J. Coll., 687. X 2.5. Oculars I, V, IV, II insert, a progressive variant, ambula- 

 cral plates simple dorsally above the 24th, which is the first compound plate (pp. 56, 174). 



95. Florida. Diam. 82 mm. R. T. J. Coll., 688. X 2.5. All oculars insert, a progressive variant. 



The development of ocular plates of Centrechinus being as above described, the adult 

 shows striking features as a matter of individual variation. All individuals considered under 

 variation were not fully grown, any more than in other species, but all here considered are 



