Chap. III. POSITION OF THE MADREPORIC BODY. 51 



it shows conclusively that Starfishes are built upon the same plan with other Echino- 

 derms, contrary to the views long entertained by Johannes Miiller. This comparison 

 to the plates of a Comatula can be carried out to its fullest extent, and is exceed- 

 ingly instructive if made with the young Comatula, of which an admirable figure 

 has been given by Professor Allman, in his valuable memoir on the prebrachial 

 stage of Comatula, in the Memoirs of the Royal Society of Edinburgh for 1863. 

 The arrangement strikes one, at once, as identical, though the plates are by no 

 means homologous. The central plate occurs in both, but the most prominent 

 plates, occupying indeed the greater part of the abactinal region of the young 

 Starfish, are the same plates which eventually develop e with others at the base of 

 the arms ; those at the angle of the arms being but little developed. It is quite 

 the reverse with Comatula, in which the arm-plates are but small, at this stage ; 

 though, according to Professor Allman, who quotes Carpenter, these small radial plates 

 eventually encroach upon the others, at the time of the appearance of the arms, the 

 rest of the calyx being formed by the five large interradial plates. I cannot 

 agree with Professor Allman in considering the central plate otherwise than as a 

 solidified homologue 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 they 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 posi- 

 tion seems to me to make this very probable. 



Position of the Madreporic Body. — There has lately been a great deal of discussion 

 among writers on Echinoderms, as to whether the madreporic body was, or was not, 

 a proper point to start from in determining the axes of the body ; Agassiz, on one 

 side, maintaining that the madreporic 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 invari- 

 ably 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, after it has 

 closed, is therefore the natural consequence of its position. The opening of the anus, 

 on the contrary, 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 madreporic body gives us the means of 

 dividing the Starfish into symmetrical halves, and of determining the position of the 



