Prof. S. Lovdn on the Structure of the Echinoidea. 431 



the young- Spatangus (see fig. 3) ; and it is clear that this is the 

 case also in the typically equally pentagonal peristome of the 

 Clypeastridai and Cassidulida3. The diameter a co passes, if 

 produced, through ambulacrum IV. and interradium 1. 



The interradia are rows of plates of the perisome, different 

 from the ambulacra in growth and movement. If, by boiling 

 in solution of soda, we carefully separate their plates in Sj)a- 

 tangus ^urpureuSj Brissopsis lyrifera, and Echinocardium 

 ovatum^ we find that many of them are furnished at their adoral 

 angles with a semilunar lamella, which projects within the aboral 

 margin of the preceding plate, which has a corresponding de- 

 pression on its inner side. Spatangus has such a lamella on 

 the third and fourth frontal and the third, fourth, and fifth 

 lateral plates 5 Brissopsis on the third and fourth frontal and 

 third, fourth, fifth, and sixth lateral plates in the hinder row ; 

 Echinocardium on the third lateral plate in the anterior row. 

 No doubt the same is the case in many Spatangidaj, perhaps 

 also in other irregular Echinoidea. Tliis is an indication that 

 the plates of the interradia are in some degree scales, although 

 they never fulfil the same functions as the scales in the peri- 

 some of the Holothuriee and Crinoidea. 



The interradia in the Echinid^ are in a high degree mutually 

 accordant ; into the peristome of these they always enter with 

 two plates, a large and a small one. It is generally only in 

 young Echinidte that the position of these can be observed. 

 Toxopneustes dr'ohachensis^ when young, constantly shows that 

 if we mark the animal's right lateral interradium with 1, and 

 consequently the unpaired one with 5, the smaller peristomial 

 plate and the larger new-formed plate close to the vertical plate 

 are found to belong to 1 a, 2 a, 3 i, 4 a, 5 a, and the larger 

 peristomial and smaller new-formed plates to 1 6, 2 5, 3 a, 4 Z», 

 5 h. It is the interradium 3, the left frontal of the animal, that 

 changes the position of the plates (see fig. 1, PL XIV.). We 

 find that such an arrangement is symmetrical on the two sides 

 of a diameter passing through ambulacrum I. and interradium 

 3, the same that is the longitudinal axis in Echinometra^ and in 

 the vertical plane of which it has the curved line of its flexure. 



Whilst in Echinoneus the interradia greatly resemble those 

 of the Echinidse in the forms and relative sizes of the plates, 

 so that even the unpaired one, although perforated by the large 

 periproctium, is still in a high degree in accordance with the 

 paired ones, there is nevertheless a remarkable difference with 

 respect to the peristomial plates, which is worth indicating in 

 order that it may be carefully investigated in young indivi- 

 duals, like so many other things in that animal, such as the 

 oblique mouth, &c. Interradia 1, 3, and 5 enter the peristome 



