OCULAR AND GENITAL PLATES. 



129 



V insert as the character, one specimen is arrested with ocular I only insert, one is progressive 

 with oculars I, V, IV and one aberrant with I, IV insert. Of Strongylocentrotus depi-essus in 

 the few specimens seen all have oculars I, V insert. 



In such a study as here undertaken it is desirable to work out as fully as possible the devel- 

 opment and variation of some one species. I have attempted to do this with Strongylocentrotus 

 drobachiensis as a good representative type and the only one of which I had the opportunity 

 to accumulate a very large number of observations. A study was made of the development 

 and variation in many localities of .33,000 specimens. In handling material and making obser- 

 vations in large numbers one is likely to make an occasional mistake, so I went over most of the 

 material a second time when a few variants were found that escaped notice on the first count. 

 The character of this species is strongly to have oculars I, V insert, but there is considerable 

 variation in all localities and also there is considerable difference in the range of variation between 

 localities. This is set forth in the text and in tabular form on p. 143. The aberrant variants 

 of this species are all given together in the table of aberrants of the Centrechinoida, p. 164. 



133 



Text-figs. 131-134. — Development of ocular jjlate arrangement in Sirongylocentroius drobachiensis (O. F. Miiller). 



131. Diam. 1.2 mm. Adapted from Lov^n, 1892, Plate 4, fig. 27. X about 22. All oculars broadly exsert; one large 

 madreporic pore only; the suranal plate fills the periproct (p. 172). 



132. Salem Harbor, Massachusetts. Diam. 4 mm. R. T. J. Coll., 767. X 22. All oculars exsert. 



133. York Harbor, Maine. Diam. 4.5 mm. R. T. J. Coll., 679. X 22. Ocular I insert (text-fig. ISl, p. 16.5). 



134. York Harbor, Maine. Diam. 7 mm. R. T. J. Coll., 680. X 21. Oculars I, V insert. 



In text-figures 132 to 134 the madreporic pores progressively increase in number and the suranal jjlatc ])ro])ortionately 

 decreases in size; no genital pores exist (pp. 171), 176). 



Taking up the development first, Loven (1892) showed that in a very young specimen all 

 the oculars are strongly exsert (text-fig. 131); a suranal plate fills the periproct; genital pores 

 do not exist, and the madreporite has one large madreporic pore instead of many as later. An 

 older specimen (text-fig. 132) has still all the oculars exsert, although ocular I approaches 

 nearest to the periproct and it compares favorably with the adult character of Strongylocentrotus 



