MUSEUM OF COMPARATIVE ZOOLOGY. 39 



There would seem to be no expressed variety of opinion on this point 

 among those who have written on the subject ; the only important 

 question which has been raised being whether the term "genital" be an 

 appropriate one to designate them, and whether their fate is the same. 

 It is important, however, for us to call attention to this fact in re- 

 gard to the so-called genitals in the two genera. While they are both 

 primary plates on interradii, the absence of primary radials in Asterias 

 has brought them to occupy a difterent position as regards the first 

 formed plates in Asterias and Amphiura. For illustration, in Asterias 

 they form an inside ring in comparison with the primary plates (termi- 

 nals); in Amphiura they form an outside ring compared Avith primary 

 plates (radials) ; they form an inside ring as compared with terminals 

 in both. The relationship of one of the genitals to the madreporic 

 opening would seem to show that the ring of plates of which this is a 

 member is the same in both cases. Ludwig regards the first plate in 

 the dorsal hemisome in the interradius of Amphiura as a madreporite. 

 If he is right, there is no doubt that the first ring of plates in the inter- 

 radii in Amphiura and in Asterias are the same. 



In considering these plates as genitals, too much importance cannot 

 be attached to the fact that no one has yet satisfactorily traced them to 

 plates with the genital openings in starfishes. It is certainly a form of 

 a priori reasoning to characterize them as genitals from the fact that 

 they are the first interradials and occupy a similar position to the geni- 

 tals of Echinoids. The one thing which we really know is that in As- 

 terias one of these first interradial plates bears a definite connection with 

 the madreporic body, and that it later occupies a similar position in one 

 of the interradii that the genitals do in the other. Is it not a jump at a 

 conclusion to suppose from this that they are necessarily genitals ? If 

 one should say that other interradials form the genitals, there are no 

 observations to show the error. That the first (orals) plates of the 

 interradii in Amphiura are genitals, we have even less to support our as- 

 sumption than in Asterias, One of these plates, according to Ludwig, 

 is perforated by the madreporic opening, and it is therefore supposed to 

 be a madreporite. All five are later consolidated in the system of plates 

 about the mouth, and bear no relationship to the genital openings. 

 Obviously these cannot be the same as the genitals of Asterias, if geni- 

 tals are homologous in Asterias and Amphiura. We are consequently 

 driven to this position : the first formed interradial plates on the abac- 

 tinal hemisome of Asterias do not enter into the formation of the mouth; 

 they occupy a position which would indicate that they are genitals, and 



