OCULAR AND GENITAL PLATES. 91 



Most species have a definite character as to whether all plates are exsert or how many 

 become insert, but in some species there is much variation, and in most species where many 

 observations have been made there is some variation. The relative frequency of typical 

 variation in selected species is shown diagrammatically in text-fig. 170, p. 153. In the tables, 

 pp. 100, 101, 142, 143, 154-164, are given in detail the relations of the ocular plates to the 

 periproct as observed in 50,000 specimens, including the leading genera and species of post- 

 Palaeozoic regular Echini belonging to the orders Cidaroida and Centrechinoida. 



As becoming insert is a progressive character with development, species in a genus that 

 have the greatest number of ocular plates insert may be considered in this respect more evolved 

 than other species which have a less number (Arbacia, Echinometra) . Also, as a matter of 

 variation, individuals that have fewer oculars insert than is characteristic of the species may 

 be considered arrested variants, and those that have more plates insert than is typical may be 

 considered progressive variants. Such variants can frequently be compared directly with 

 related species or genera where the fewer or greater number of oculars insert is a typical specific 

 character (Centrechinus). Specimens of a given species from different localities present often 

 quite striking differences as regards the number of plates which are insert, those from one 

 locality having typically fewer oculars insert than those from a different locality. Such varia- 

 tion with locality may well be considered as indicating incipient species, as, where there is a 

 difference, specimens from one locality must be more progressive or less progressive than 

 those from another {Cidaris affuiis, p. 100, Tripneustes esculentus, p. 161, Stromjylocentrotus 

 drobachiensis, p. 143, and Echinometra lucunter, p. 163). 



The number of oculars insert has been spoken of by previous writers as if it were a con- 

 current of age, and the largest specimens had the most oculars insert. My observations are 

 directly opposed to this view. All the evidence goes to show that the full number of oculars 

 that are to become insert are developed early in the life of the individual, and apparently later 

 no change in this respect takes place. A series of specimens half the mature size or larger may 

 in most species be safely accepted as showing the mature characters as regards oculars. This 

 is on the basis of observations on 11,500 specimens of Strongylocentrotus drobachiensis, all from 

 one locality, Dumpling Islands, the specimens varying from very young to adult, and all meas- 

 ured and tabulated as later described. With few exceptions it was found that the larger 

 individuals in a species are typical as regards ocular plates, and that variations, both arrested 

 and progressive, are more frequent in smaller individuals, often half grown as regards size. 



In many cases it has been found that my observations of the typical number of oculars 

 insert do not agree with the published descriptions of the species. This is probably due to the 

 fact that variation was not considered, and descriptions were drawn from a specimen or speci- 

 mens that happened to be variants. It is not enough to consider how many ocular plates 

 become insert, but which ones is an important matter. In this there is a certain difference 



