32 STRUCTURE OF THE OYSTIDE^. 



It is quite clear from the above extracts that the only reason upon 

 which von Buch founded his opinion was, that the Cystideae are 

 without arms. From the expressions in italics, there can be little 

 doubt that if he had examined specimens with arms well preserved, 

 such as some of those figured in this decade, the idea of an ovarian 

 aperture would scarcely have occurred to him ; in fact it would, to 

 use his own words, have appeared to him ' useless.' Since therefore 

 it is now beyond all question that the Cystidese had true crinoidal 

 arms, the only reason that ever suggested to any one the necessity 

 of a genital aperture has no longer an existence, and the question 

 remains exactly as it was before the publication of his memoir on 

 the Cystidese. 



If then we should no longer regard it in the light of a genital 

 opening, the next question to be decided is, What was its function ? 

 I believe that in those species which were provided with an anus, 

 it was the mouth, and in the others both mouth and anus. The 

 following are my reasons : — 



1. In EchinosphcBrites aura?itium, Caryocystites granattwn, and many 

 others, there is a well-defined anal aperture ; and if the ambulacral 

 orifice were also the mouth, then it would follow that the lateral 

 opening had no function. The same would be true of the genera 

 Echino-encrinites, Ajriocystites and Callocystites, all of which are said 

 to exhibit anal pores near the summit. But if the apical opening 

 be an ambulacral orifice only, then there is no place for the mouth 

 but in the lateral aperture. 



2. The position of the organ in question is not inconsistent with 

 the idea of its being an oral aperture, because in many of the 

 Echinodermata we find it not only in the side but even on the under 

 surface of the body. Thus in the Echinidae the mouth is in the under 

 side, and the anus either in the very highest part of the body or in 

 the side. The Star-fish always crawls with his mouth under, and so 

 do the Ophiura, Euryale and Comatula. The fact therefore that the 

 lateral aperture of the Cystideae is lower down than the anus is no 

 proof that it is not the mouth, but directly the reverse, since it is 

 the natural position for the organ in the class. 



Even among the true Crinoids, we have at least one species with 

 the mouth in the side, as represented by the following wood-cut, 

 taken from the figure in plate C. of Pictet's Atlas. The genus has 

 five basal plates and five radials; the mouth penetrates the large 

 inter-radial, and there appears to be five ambulacral orifices. There 

 can be no doubt but that this species is a true Crinoid, as the rays 



