(HTLAR AND GENITAL I'l.ATKS. 



129 



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

 with oculars I, V, IV and one aberrant with I, IV insert. Of Strongylocentrolu* depressu* 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 us po.--.iblr the devel- 

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

 drobachiensis as a good representative type and the only one of which I h;id tin- opportunity 

 to accumulate a very large number of observations. A study wa- Mimic of the dcvi-lopMicut 

 and variation in many localities of 33,000 specimens. In handling material and makinn " : 

 vations in large numbers one is likely to make an occasional mistake, so I went over moat of th> 

 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 variat ion bet ween 

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

 of this species are all given together in the table of aberrants of the Ontrechitioida, p. I HI. 



134 



TEXT-FIGS 131-134 Development of ocular plate arrangement in StrongylocenlTolw drdbachifnti* I M < : I : 

 131. Diam.l.2mm. Adapted from Loven, 1892, Plate 4, fig. 27. X about 22. All ocular, t 

 madreporic pore only; the suranal plate fills the i>eriproet (p. 172) 



132 Salem Harbor, Massachusetts. Diam. 4 mm. R. T. J. Coll., 767. 



133 York Harbor, Maine. Diam. 4.5 mm. II. T. J. Coll., 679. Or,,l:,r I ins,r, , ,,*,-. 1 



134 York Harbor, Maine. Diam. 7 mm. R. T. J. Coll., 680. 



In text-figures 132 to 134 the madreporic pores progressively inm,r in n.n..lM-r i 

 decreases in size; no genital pores exist (pp. 170, 176). 



Taking up the development first, Loven (1892) showed that in a very you,,* spmn. all 

 the oculars are strongly exsert (text-fig. 131); a suranal plate fill* 

 do not exist, and the madreporite has one large madreporic pore instead 

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

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



