OCULAR AND GENITAL PLATES. 131 



had but just taken on this position. Specimens (82) 4 to 5 mm. in diameter have 4 % with all 

 oculars exsert and 96 % with ocular I only insert. All the specimens up to this age lack genital 

 pores. Specimens (800) 5 to 10 mm. in diameter have 0.1 % with all oculars exsert, 46 % with 

 I only insert, and 52 % have taken on the species character of oculars I, V insert. This shows 

 a very rapid development, for while no specimens under 5 mm. have the bivium insert, more 

 than half have it insert between 5 and 10 mm. While in specimens under 5 mm. all genitals 

 are imperforate, of those between 5 and 10 mm. only 128 out of 800 have all genitals imper- 

 forate. In addition, however, 44 specimens were noted in which only part of the genitals were 

 perforate (p. 170). While in this series there are no regular progressive variants, there is 1 % 

 of aberrant variants, the usual number for any age in this species. As the aberrants are 

 all grouped together in one tabulation, it is desirable in the series to give the details, especially in 

 these younger stages. In this 5 to 10 mm. series the aberrants are: one specimen with oculars 

 I, II insert, four with I, IV, and five with I, V, II insert, these last being the only cases of three 

 oculars insert found of this age. One specimen has oculars I, V, IV, III insert and II exsert 

 (text-fig. 149). This is of interest as having four plates insert at this very early age and is an 

 extremely rare aberrant variant for the species, since only two cases have been found in the 

 whole 33,000 examined. 



The next older series of 10 to 15 mm. in diameter (2,000 specimens) has progressed markedly. 

 No specimens have all oculars exsert, 19% have I only insert, and 80% have I, V insert. 

 The progressive variants here appear and 0.3 % have oculars I, V, IV insert. Only six speci- 

 mens of this age had all the genital pores wanting, and no larger individuals were seen lacking 

 all the pores. This stage is much more evolved than the next smaller series. Of aberrants the 

 number is 1 % as usual. Two specimens have ocular IV only insert, a very rare variant; four 

 have oculars I, IV, and six have I, II insert. Fifteen specimens have oculars I, V, II insert, a 

 number much exceeding those that have I, V, IV insert at this stage. This is of interest as 

 showing in these early stages a preponderance of oculars I, V, II insert, whereas in the species 

 as a whole, when three oculars reach the periproct, oculars I, V, IV insert preponderate over I, 

 V,II. 



Passing to the next size, 15 to 20 mm. in diameter (1,400 specimens), only 7% have ocular 



I alone insert; 92% have I, V insert, a fairly close approach to the adult percentage; and 0.4% 

 have oculars I, V, IV insert, relatively a little more than in the earlier stage. The aberrants 

 are the usual 1 %. Of these, one specimen has oculars I, II insert, one has V, IV, and one V, 



II insert; six specimens have I, V, II insert, this number still preponderating somewhat over 

 the proportion of specimens with oculars I, V, IV insert at this stage in development. 



The next size, 20 to 30 mm. in diameter (1,250 specimens), is the last of the developing 

 series and makes a close approach to the adult condition. Only 5% have ocular I only insert, 

 93% have I, V, 1% have oculars I, V, IV, and 0.1% have oculars I, V, IV, II, insert. This 



