STRUCTURE OF THE CYSTIDE^. 



21 



of the palEeozoic Crinoids (to be mentioned in the next paragraph) 

 partake also of the nature of ovarian or genital apertures. 



The structure of the Star-fishes and recent Crinoids only agrees in 

 these respects : that both groups have the ambulacra outside of the 

 body, and the ambulacral orifice and mouth combined in a single 

 opening. 



III. The PalcBozoic Crinoids. 



In at least a great many species of the paleozoic Crinoids we 

 find an arrangement so different, that it almost justifies their 

 separation into a division distinct from the recent forms. The 

 structure of the arms is precisely the same, and there is not the 

 least doubt that their functions were also to support the ambulacra 

 and reproductive organs, as in Pentacrinus and Comatula. In the 

 ventral surface however, or in the circular space surrounded by 

 the arms, there is only one large opening, but no grooves radiating 

 from it to those of the arms. To shew more clearly the difierence 

 between the recent and extinct species in this respect, I have 

 constructed the following diagrams : — 



Fig. 1. Recent Crinoid. 



Fig. 2. Palseozoic Crinoid. 



Fiofure 1 is a dia2;ram of the ventral surface of a recent Crinoid 

 with the mouth, m, situated in the centre, and the ambulacral 

 o-rooves radiatino; from it to the arms. The anus, a, is situated 

 between the centre and the margin, and in some of the species of 

 the genus Comatula it forms a tubular projection of several lines in 

 length. 



Figure 2 is an ideal representation of the ventral region of a 

 palaeozoic Crinoid, with onl}^ one opening, m a, but with no grooves 

 leading from it to the arms. 



