IN THE MOKPHOLOay OF THE CYSTIDEA. 31 



of a Crinoid is a point of some interest, and may' prove to be of 

 use for purposes of classificatiou. I would likewise assign an 

 excretory function to Yolborfcli's organ in GlyptosplicBra and tlie 

 fourth opening in Aristocystis (PL I. figs. 12, 13, 15), and it 

 is quite possible tbat Volborth was correct in regarding the 

 former as a madreporite. At any rate it occupies the same 

 position, relatively to the genital pore, the mouth, and the anus, 

 as the madreporic opening has in those Holothurians in which 

 it retains its primitive connection with the exterior ; and we 

 must not lose sight of the possibility that there may have been 

 Cystids which had a distinct external madreporic opening in early 

 life, though it subsequently closed up, as is the case in many 

 Holothurians. 



Thus, for example, it may have remained permanently open in 

 Glyptosplicera and have closed in Protoct^inus, the nephridial duct, 

 if such existed, perhaps acquiring a communication with the ex- 

 terior through a genital pore. I do not wish to be understood as 

 implying that I fully believe this to be the case. But in endea- 

 vouring to throw some light upon the morphology of these ancient 

 forms, one must not lose sight of the possibilities of explanation 

 afforded by their recent representatives. 



The analogy of Glyptosphcera and Aristocystis would seem to 



indicate that when there is a distinct opening betioeen the mouth 



and anus, as in Sphcdronis, Protocrinus, and Proteocystis, it 



should be recognized as genital ; though we might, of course, 



look upon it as excretory, and assume that there was a common 



oscular orifice for the anus and genital ducts, as I have done for 



Agelacrinus and Caryocrinus. I must confess that I am rather 



inclined to take this view of Spli(sronis *, which has a large anal 



pyramid just behind the mouth, and a minute valvular opening 



close to the left posterior ambulacrum (D) which may very 



well have been excretory in function. It is quite possible 



also that the third opening of Caryocystis, Echino splicer a, and 



Megacystis was nephridial or madreporic, rather than genital f 



though it might, of course, have served both functions, as 



suggested above. This possibility is to some extent supported 



by the embryological fact that the primary water-pore of Echino- 



derm larvae is situated in the anal interradius, which also contains 



the chief part of the labial plexus and ovoid gland of a Crinoid, 



* See Angelin's ' Iconographia,' tab. xi. 

 t See postscript, infra, pp. 49, 50. 



