POSITION OF THE MADREPORIC BODY. C3 



Professor Allnum, who quotes Carpenter, these small radial plates event- 

 ually encroach upon the others, at the time of the appearance of the 

 arms, the rest of the calyx being fonned by the five large interradial 

 plates. I cannot agree with Professor AUman in considering the central 

 plate otherwise than as a solidified homologi e of the basalia of the other 

 Crinoids figured by him; the only difference being that in some cases 

 the plates composing this piece are soldered together, as in Comatula, 

 while in others tliey are kept distinct, as in Coccocrinus, and the like. 

 From the peculiar way in which young tentacles are formed in Starfishes 

 may not the strange toothed plates noticed by Professor Allman, at the 

 base of the tentacula (or cirri, as he calls them), be young tentacles? 

 Their position seems to me to make this very probable. 



Position of the Madreporic Bocl/j. — There has lately been a great deal 

 of discussion among writers on Echinoderms, as to whether the madre- 

 poric body w^is, or was not, a proper point to start from in determining 

 the axes of the body ; Agassiz, on one side, maintaining that the madre- 

 poric body was constantly in the same relation to the different parts of 

 the Echinoderms, while Miiller, Cotteau, and Desor have warmly opposed 

 this view. The mode of formation of the madreporic body seems to me 

 to decide this question in favor of the former view. The madreporic 

 body is invariably formed on the left water-tube of the Brachiolaria, and 

 is placed, during the development of the Starfish, at the angle of the 

 upper arm. The future position of the madreporic body opposite the 

 third arm of the open pentagon is therefore, after it has closed, the nat- 

 ural consequence of its position. The opening of the anus, on the con- 

 trary, has no such clear and precise relation to the middle arm. At any 

 rate, however this may be, one thing is perfectly apparent, viz. that 

 the madreporic body is always placed in the suture of the terminal arms 

 of the pentagon, which brings it opposite the third arm. Thus the mad- 

 reporic body gives us the means of dividing the Starfish into symmetrical 

 halves, and of determining the position of the odd arm. The case of the 

 Echinometradae and SalenidaB is constantly quoted to show that the mad- 

 reporic body is not connected with any definite axis. But might it not 

 be that a stage which is embryonic in the young Starfish — viz. that 

 preceding the closing of the actinal and abaetinal areas — is probably re- 

 tained in those Echinoid families in which the process of closing is not 

 completed ? And may not the unsymmetrical position of the madreporic 



