Prof. S. Loven on the Structure of the EcTiinoidea. 439 



the vertex, tliey are placed between the vertical plates and the 

 interradia, as in MeUita, or, as in Clypeastevj entirely in the 

 latter and separated from the vertex by their last two or three 

 plates. Cotteau long ago made the important observation 

 that an Echinid also, namely Goniopygus^ has the genital 

 pores outside the vertical plates, near their apices*. But in 

 all the Echinida3 every one of the five vertical plates bears its 

 genital pore, and the madreporite is confined to one of them 

 only, namely 2, the right anterior one. It cannot be doubted 

 that the madreporite and sand-canal are carried to this position 

 in this way — that the intestinal canal, which, in the irregular 

 Echinoidea, has its anal orifice surrounded by the periproctium 

 in tlie unpaired interradium and only in that, but there ter- 

 minates at any point from the neighbourhood of the mouth 

 till it cuts through the circle of the vertical and ocellar plates, 

 opens in the Echinidse in the middle of that circle, which closes 

 round it. While the moutli, Avhich opens earlier, has in all 

 Echinoidea the same position with relation to the ambulacra 

 and interradia, and its peristome independently formed regu- 

 larly by the same p kites fitted for the purpose and in a definite 

 order, the anal apL'.i;ture has a highly variable position, sm*- 

 rounded by eroded plates, in which it occupies during growth 

 a gradually increase'd space. 



In a jowngToxopneustesdrohachensis of 5 millims.the vertical 

 plates form a closed circuit, each over its interradium, and in their 

 reentrant angles the five plates which bear the eyes are placed 

 equally regularly. This is also the case in adult individuals of 

 Echinus J Sj)ha^rechmus, and Psammechmus, and also in the 

 Salenidse. But in Toxopneustes and most others this primor- 

 dial and normal arrangement is soon disturbed. The eye- 

 plates of the bivium are gradually pushed into the circle on 

 both sides of the vertical plate of the unpaired interradium 5, 

 between this and 1 on the right side and 4 on the left. It is 

 eye-plate I. that first reaches the inner circumference, and 

 next eye-plate V., as in most genera, such as Loxecldnus^ Lyt- 

 echinus^ Heliocidaris^ Tripneustcs^ Boletia^ SahnactSj Echino- 

 cidaris^ Acrocladia, and Echinometra ; in Amhlypneustes and 

 Mespilia they come quite near it. Of the eye-plates of the 

 trivium, IV. approaches the inner circumference, which it 

 reaches in many ; II. also approaches it, but in a less degree ; 

 and III., the eye-plate of the unpaired ambulacrum, is always 

 distant from it. In Diadema^ on the other hand, all the eye- 

 plates are seen more or less completely to touch the anal 



* " Echinides fossiles du D^partement de la Sarthe," pp. 152, 154, pi. 26. 

 fig, 2, and pi. 27. fig. 25 ; " Echinides fossiles du DtSpartement de I'Vonne," 

 ii. p. 50, pi. 52. fig. 14 : Bull. Soc. Gcol, Fr. 2" s^r. xvi. p. 102. 



