ECHINID^E. 235 



near the peristome five (sometimes four or even three) pore-pairs in each arc; 

 at the ambitus and above there will be six and near the ocular plate, there will 

 often be five or only four. In a really young specimen of the same species there 

 will be four (or only three) pairs near the peristome, five at and above the ambitus 

 and six near and adjoining the ocular plate. In the Echinidse and Temnopleur- 

 idse, there is no change in the number of elements in each plate as one passes 

 from the peristome upward but the relation between them becomes more and 

 more simple and obvious as one passes from the mid-zone to the ocular. This 

 increasing simplicity is most marked in forms like Holopneustes and Tripneustes. 

 Finally, it should be remembered, that any individual plate is liable to mal- 

 formation and to variation from the typical condition, so that plates with only 

 four or five elements are occasionally found intercalated between those having 

 six or even more. 



Immature specimens are often a source of difficulty and error. The absence 

 of genital pores is one of the most obvious evidences of immaturity but unfortu- 

 nately these pores appear long before maturity so that their presence is no 

 criterion of age. Disproportionately large abactinal and actinal systems are 

 youthful characters, while the appearance of the periproct, the pore-pairs, and 

 the spines often show whether a specimen is mature or not. The identification of 

 young Echini is often very difficult and it is frequently impossible to distin- 

 guish the young of allied species or even of allied genera, unless a series of speci- 

 mens showing growth changes is available for comparison. 



No reliable system of classification permits the certain identification of 

 isolated, immature specimens. One of the very few conveniences of using the 

 pedicellarise as the basis of classification in Echini, is that they are essentially 

 the same in the youngest specimens in which they are found, as in the adult. 

 In reality however this is an important argument against their validity for 

 systematic purposes, for it can hardly be questioned that a character which 

 appears full-fledged in early youth and undergoes no change in ontogeny, has 

 no phylogenetic significance. The occasional convenience therefore of using 

 the pedicellariae in identifying young Echini should not be construed as evidence 

 of their systematic value. 



