412 PROF. F. J. BELL ON THE ECHINOMETRTD^. [Mat. 15, 



Turning now to the mode by which these pores come to be so 

 arranged in the adult, it will be well to recapitulate shortly the pre- 

 sent state of our knowledge concerning it. This knowledge has, 

 within the last decade, been very considerably advanced by the ela- 

 borate and beautiful researches of Prof. Lov^n^ I shall depart 

 from a strict following of his account only in using the term 

 " secondary plate " as a translation of his " plaque composee." As 

 has been already pointed out, these secondary plates, when developed, 

 are made up of three or more primary plates. Now, •' the primary 

 ambulacral plates of the Echinidee are either entire (that is to say, they 

 occupy the whole of the distance between the interradial area and 

 the median suture of the ambulacrum'-', or, in other words, extend 

 from the interradial area as far as the middle of the entire plates), or 

 they end by a more or less sharp point. The major primary plates 

 of the peristome forming the series la . . . V6, most often consist, in 

 very young individuals, of a first entire primary, of a median pri- 

 mary' half plate, and of a third entire primary plate." In an 

 appended table the learned author shows the arrangement of the 

 entire and half plates in the several secondary plates of the corona 

 of a small specimen of Toxopneustes (^Strongylocentrotus) drobachi- 

 ensis. The fourth or fifth of these has Iwo complete and three 

 half primaries, as is shown by the formula — 1, (2, 3, 4), 5. 



Next we come to the mode of growth of these different primary 

 ])lates. "Near the aboral edge of a complete composite plate there 

 is deposited the first primary plate of tlie new plate, then the second, 

 and soon. All the primary plates, and even the half-plates, are 

 primitively entire plates ; that is to say, they extend from the inter- 

 radial area as far as the median suture of the ambulacrum^. Later 

 on, and during the period in which the entire collection of primary 

 plates constituting the composite plate goes on enlarging, and even 

 before it is completed by the last primary plate, the intermediate 

 plates cease to grow ; and while retaining their position on the edge 

 of the ambulacrum, beside the interradial area, they shrink at their 

 extremities, which become separated from the median suture. They 

 consequently become cuneiform. Of these intermediate plates the 

 smallest is always that which is formed first ; those which are formed 

 later are always successively larger, whence it follows that the whole 

 group of intermediate primary plates takes the form of a triangle, 

 the apex of which, in the middle of the composite plate, only con- 

 sists of the projecting extremity of the latest of them. It clearly 

 results from all this, that these intermediate plates are in no way 

 of a more recent origin than the others, that they are neither 

 secondary nor intercalated, but that they are successively formed, 

 after the first entire plate, and before the last ;" and Johannes 

 Miiller taught just the same. 



The formation of the secondary arcs is no less clearly explained, 

 and is shown to be primarily due to the compression from above 



1 ' Etudes sur les Ecliinoidees,' especially pp. 21 et seq. 



' As in Cidaris. ^ The italics are mine. 



