THE PERISTOME. 79 



or, if, by chance, a plate is wider than usual, as in the tetramerous Arbacia lixula figured by 

 Verrill (1909), then again the tubercles are their usual distance apart, but there are more 

 tubercles to a plate than is typical of the species (pp. 40, 47). 



THE PERISTOME. 



The peristome is closely associated with the corona, and is next considered. Tin- peri- 

 tome, which is the tissue lying between the mouth opening and the basicoronal plates, present - 

 various characters in different groups of Echini, and these characters fall into six quite distinct 

 types as described below. 



The peristome in almost all Echini is more or less extensively covered with plates. Tin-H- 

 are in regular Echini ambulacral plates bearing tube-feet, either in one or in many rows. With 

 these there may be in addition what I would call non-ambulaeral plates, which do not boar 

 tube-feet. Such non-ambulacral plates, where they occur in the Perischoechinoida and Cid.-i- 

 roida, occupy an interradial position; or, where they occur in the Centrechinoida, they occupy 

 also a radial position, being a more or less generally distributed series of plates (Toxopneu 

 text-fig. 57). In spatangoids there are no ambulacral plates on the peristome, but non-ambula- 

 crals only. These are generally distributed over the area. I have adopted the term tin- 

 ambulacral for such plates because, though they may be interradial only (Cidaridae), they may 

 also have a general distribution, and they have no relation to the interambulacra of the corona. 



The most ancient type of peristome known is that represented by Bothriocidaris (t<-\t- 

 fig. 40) in which there are two continuous rows of ambulacral plates, a structurally primitive 

 character. The first row around the mouth can fairly be considered the primordial ambulacral 

 plates, and, as shown in B. archaica (Plate 1, fig. 1), this row follows LoveVs law of la, II</. 

 III6, IVa, Vb large and 16, 116, Ilia, IV6, Va small. Above the second row the corona pn>|>cr 

 begins with a row of five interambulacral and ten ambulacral plates. The line of separation 

 between the peristome and the corona is not so strongly marked as in most typos, but it is 

 comparable to that of young Phormosoma (text-fig. 41), as pointed out by Mr. Agassi/, i 1<H4. 

 p. 79). It is quite probable that the second row of peristomal plates in Bothriocidaris \\.t- 

 derived by flowing down from the corona, but on the other hand there may have been two n.\\> 

 of peristomal plates formed before the development of the corona. In Phormosoma at a \vry 

 young stage (Plate 3, fig. 9), as shown by Mortensen (1904, p. 54), there is only one row of p>ri- 

 stomal plates, whereas a little later (text-fig. 41; Plate 3, fig. 10), as shown by A. Agjissiz. there 

 are. two rows of peristomal plates almost exactly as in Bothriocidaris. a- Mr. Agassiz (I 

 p. 79) pointed out. The peristome of Bothriocidaris makes also very close approach to the 

 character of the young in cidarids (Plate 2, figs. 1, 6) and young Stroimyloecntn.t..- t.-M-fig. 

 49), also other young Centrechinoida. It differs from these early stages principally in having 



