40 BULLETIN OF THE 



one of them is early brought into connection with the madreporic open- 

 ing. The plates of Amphiura which are in an homologous position ap- 

 pear not to have any relation to the genitals of the adult. One of them 

 is a madreporite. The ultimate fate of both in Asterias and Amphiura 

 is conjectural, but probably different. 



In other words, while the genital of a starfish may be brought by its 

 growth into intimate union with the end of the stone canal or with the 

 opening of the madrepoi'ic tube, it may readily be seen that in Amphi- 

 ura the oral, a plate of totally different homology, might have the same 

 relation without the same homology. It would not make these plates 

 homologous, i. e. genital and oral. Even if in Brisinga, with its ophiu- 

 ran and starfish affinities, the madreporic opening is found on the odon- 

 tophore, it is not necessary to regard the odontophore as a genital. 

 The whole thing of course hinges on the acceptance or denial of the dic- 

 tum that the presence of the madreporic orifice means homology of the 

 plate through which it opens. While many naturalists in whose opin- 

 ion we have the most confidence hold that it does, that the existence of 

 the madreporic opening in a plate settles its homology once for all, to 

 me it leads in some cases to insurmountable difficulties. It seems to 

 me that the objection to the homology of the genitals of Asterids with 

 the orals of Amphiura is well taken, if its defenders adopt the theory 

 that the odontophore is an oral ; for the genitals of starfishes are cer- 

 tainly not odontophores, even if in Brisinga one of the odontophores 

 bears the madreporic body, or is a madreporite. The mistake seems to 

 me just here : the madreporite may not be one of the genitals, or ho- 

 mologous with them, even in Echinoids, but is rather to be considered a 

 separate plate, which may have connection with one of the heart-shaped 

 bodies which form the odontophore. Ludwig was, I think, right in a 

 comparison of the odontophores of Asterids with the orals of the Ophiu- 

 rids. A. Agassiz has expressed the thought that the oral, or the plate 

 we now call the odontophore, is an interbrachial, an homology which is 

 sound, in the light of the development. The fact that in its early for- 

 mation that genital plate which is nearest the madreporic opening forms 

 wholly independently of the structure in question, has a meaning. In its 

 early condition this plate is not penetrated by the tube in Asterias; but 

 it is only later, after the brachiolarian notch has been considerably re- 

 duced in size, that the spicules begin to grow around the opening to en- 

 close it. It is not until the stellate form has been assumed, and the 

 arms have reached a considei'able development, that the madreporic 

 plate appears. It may be said that all the primai-y plates of the body 



