Prof. J. Miiller on the Structure of the Ecliiiiodcrnis. 5 



remarked,, that its perforated area (Porenfeld) in Schizaster ca- 

 naliferus extends from the right genital plate and from the im- 

 mediate neighbourhood of the right genital pore. A short time 

 ago I had occasion to demonstrate this by internal dissection. 

 The canal which passes from the madreporic plate to the circular 

 canal of the ambulacral vessels, the analogue of the sand-canal 

 of the AsteriadcB, is inserted in fact, in Schizaster canaliferus, into 

 the circular canal between the right anterior and right posterior 

 ambulacral canals, corresponding therefore with the right poste- 

 rior inter-radius. In the Clypeasters the posterior extremity is 

 also determined by the position of the anus ; but in this divi- 

 sion of the Sea-urchins the madreporic plate, on the other hand, 

 lies exactly in the centre between the genital apertures, which 

 are not carried by special plates. Singularly enough, however, in 

 this case, the canal which passes from the madreporic plate to 

 the circular canal, as well as the short thick heart which lies 

 close to it, is so disposed, that its position corresponds, not with 

 that of the anal inter-radius, but with that of the right posterior 

 inter-radius. From all this it follows, that the anus as well as 

 the madreporic plate may be intcr-radial ; that sometimes the 

 one and sometimes the other may pass out of its inter-radial 

 space into the centre ; that the inter-radius of the madreporic 

 plate may be distinct from that of the anus ; that the madreporic 

 plate, even when it lies between the genital pores or at the pos- 

 terior extremity of the apex, may be reduced to a lateral genital 

 plate; and that it is united, not with the posterior part of the 

 circular canal, but with a lateral part of it by means of the 

 ' sand-canal j^ finally, that no one of the five inter-radii is inva- 

 riably that of the madreporic plate, but that this may be seated 

 in the most widely different inter-radii, sometimes on the right 

 and sometimes on the left side. 



We may now compare the azygos radius in those Echinoderms 

 in which it is easily determinable, as in Echini and Holothuria, 

 and it at once appears that an anterior radius corresponding 

 with that of the Spatangidce cannot universally exist in the 

 Echinodermata. In the Holothuriadce with an ambulatory disc, 

 the anterior and posterior extremities and the bilateral sym- 

 metry also are indisputable. The azygos radius, however, is 

 here directed, not from the mouth upwards and forwards, so 

 as to lie in front of and above it, as in the Spatangidce, but 

 takes exactly the opposite direction — from the mouth backwards 

 and downwards — and lies beneath and behind the mouth ; the 

 azygos radius is here the middle ventral radius and the ambu- 

 latory disc is formed by three radii, while on the dorsum there 

 are only two. 



Whether now is the true ventral surface, that of the Spa- 



