KONGL. SV. VET. AKADEMIENS HANUL. BAND. 19. N:0 7. 27 



thening ridges are no longer observed in specimens but. little larger, in which the peri- 

 stome has begun to change form, and are not to be recognised in mature specimens. 

 The nearly circular orifice of the alimentary canal has just opened in the centre of the 

 buccal membrane. In this there are deposited a number of calcified laminaj, in three 

 concentric series, the outermost consisting of by far the largest, ten in number, placed two 

 and two against the interradia, while the median, sub-pentagonal series is composed 

 of about fifteen much smaller laminte, and the innermost, encircling the opening, is 

 a chain of numerous minute particles of reticular tissue. This calcified incrustation 

 may possibly be found to pertain to the interradial system. 



Thus, it is seen, in the true Echinids as in the Spatangi, at a very early age, 

 and at the time when the skeleton assumes its final form, the five ambulacra one and 

 all are constituent parts of the peristome, each by both the two plates of its first 

 pair. And in the adult this is the general rule, found to hold good everywhere in 

 the whole of hitherto known Echinoidea, without exception. Normally the first ambu- 

 lacrals alternate with the first interradials, which reach the peristome Avith two plates 

 or with a single compound plate. But between the Dentiferous and the Edentate forms, 

 these last as far, at least, as we know them at present, there is this difference that, in the 

 former, the peristome, central and perfectly circular from the beginning, and divided 

 etjually among the five ambulacra as likewise among the five interradia, remains so with 

 very little change during life, whereas in nearly all the Edentates it is at first sub-central 

 and pentagonal, but during growth is drawn forward, and becomes transverse and more 

 or less labiate. In the Spatangida^ these changes are accompanied with very unequal 

 alterations in the constituent parts, by the enlargement of the trivious peristomals, in 

 a few cases to the exclusion of the interradials 1 and 4, or even 2 and 3, the bivious 

 ambulacrals retaining more or less their wedge-shaped outline; by the first interradial, 

 5, 1, advancing and arching, so as to form a prominent labrum; and by the oesopha- 

 geal opening becoming, from an almost circular aperture, a transverse fissure. '). 



From this a single form alone among the Spatangidfe is an exception, the Pahc- 

 ostoma rairabile Gray, PL XVI, Jig. 184 — 196.^). In all essential points it is a true 

 Prymnadete Spatangean, Avith an oviform body, inflate posteriorly, the back being 

 raised behind the middle. The peristome is somewhat anterior, at the four fifths of 

 the entire length, rather small, slightly sunk, regularly pentagonal and elabiate even 

 in the adult, being composed of five broad interradials of similar form, constituting 

 its sides, and five equal pairs of ambulacrals at its angles. On the inside, fig. 188, it 

 is strengthened by a pentangular ridge running parallel to the margin, and correspon- 

 ding to that described above in the young of Echinocardium flavescens 0. F. M., but 

 here observable, at least, in the half-grown animal. The pentangular stoma is filled, 

 not with a pliant membrane incrusted with calcified luminal but with five flat, equal, 

 triangular and contiguous valves, meeting together centrally, each of them composed 

 of an external basal part articulating with the corresponding interradial side of the 



>) Etudes, p. 14, pi. Ill, fJK. .32, 33— 37; V, lis?. 4(5, 47; VII, fig. 61, 07. 

 •-) lb. p, 16, 50: pi. XXXII, fig. 197—199. 



