OCULAR AND GENITAL PLATES. 133 



1 % as usual. Of these, one specimen has ocular II only insert, a very rare variant, seen also 

 in one specimen from Frenchman's Bay, but otherwise in only six other sea-urchins, namely in 

 Slrongylocentroius lividus and a Microcyphus maculatus. Three specimens have oculars I, IV, 

 one V, IV, two V, II, and eight I, V, II insert. 



Before considering the developed series from Dumpling Islands, the variation of the species 

 as a whole may be briefly stated. Omitting the developing series as above described, 27,417 

 specimens have been examined from various localities. Of these, the oculars I, V are insert 

 in 25,766 specimens, 94%, as a strong species character (text-fig. 137). As an arrested variant 

 only one specimen, 0.004%, has all oculars exsert (text-fig. 135), as in the young. It is rather 

 remarkable that this variant should be so very rare, as it is the character in two species of the 

 genus. The arrested variant of ocular I insert is rather common, existing in 746 specimens, 

 or 3% (text-fig. 136). When one plate alone is insert in development or as an arrested 

 variant, it is almost always ocular I, for while in the total recorded in this species 1,758 cases 

 of ocular I insert occur, there are only eleven cases of ocular V alone insert and these all occur 

 in adults, as shown in the table. As progressive variants, 545 specimens, or 2% have oculars 

 I, V, IV insert (text-fig. 138). Very rarely, in eight specimens, or 0.03%, oculars I, V, IV, II 

 are insert (text-figs. 139, 139a). This is the extreme range of regular variants found in the 

 species. Of adults there are 340 aberrants, or 1.2%. 



The aberrant variants in the species reach 1.2% and, as shown in the table of aberrants 

 (p. 164), cover a wide range of characters, but more than one half of the total are cases of 

 I, V, II insert. The principal aberrant variants are shown in text-figs. 140 to 149, which figures 

 also serve to show the principal aberrant variants of the order Centrechinoida. Taking up the 

 aberrant variants of Strongylocentrotus drobachiensis, and here including the whole lot of 33,000 

 specimens, both young and adult, we find that very rarely ocular IV alone may be insert (text- 

 fig. 140) ; this was found in only three cases, while still more rarely (two cases) ocular II alone 

 was insert. In no case was III alone insert. Oculars I, II alone are occasionally insert (29 

 cases) (text-fig. 141). This character is typical of Gymnechinus pulchellus (text-figs. 177, 178). 

 As a left-handed equivalent of this last, oculars V, IV alone may be insert, 33 cases (text-fig. 142). 

 This character is more frequent in the Arbaciidae, as there noted. An ocular occasionally fuses 

 with a genital (text-fig. 143). This has been found in 103 cases and may occur between any of 

 the oculars with either of its adjacent genitals. Oculars I, IV may be insert (text-fig. 144). In 

 the great majority of the 81 cases of this aberrant seen in this species, genitals 4, 5 are fused as 

 in the figure, mechanically shutting out ocular V, so that it may reasonably be considered 

 an imperfect I, V, IV insert. Oculars V, II may be insert (text-fig. 146), and in all the 34 

 cases of this variant seen genitals 5, 1 are fused, mechanically shutting out ocular I, so that 

 this variation may be considered an incomplete I, V, II insert. Oculars I, V, II may be 

 insert, 215 cases (text-fig. 145), and this is the commonest aberrant variant found in the species 



