CRINOIDEA: CYSTOIDEA : BLASTOIDEA. 577 



Antiambulacral nervous system, Milnes Marshall, Q. J. M. xxiv. 1884. 

 Process resembling copulation in Comatula, Jickeli, Z. A. vii. 1884 (A. N. H. 

 (5), xiv.). 



CLASS CYSTOIDEA. 



Extinct Pelmatozoa, either sessile, or furnished with a short stalk. 

 The body is ovate or globular, and covered with polygonal plates rarely 

 arranged with regularity. The stem-joints are round and ring-like, the 

 median canal being large. It is pointed below, and the joints are some- 

 times telescoped one into the other. The mouth is elevated and central, 

 and five ambulacral grooves may be traced radiating from it in many 

 instances. A tube covered by plates probably represents the anal tube, 

 and a third aperture, sometimes present, a genital or ovarian opening. 

 Arms, two to five in number, are generally present, but are feeble and 

 unbranched. They may carry pinnulae, and the ambulacral grooves may 

 be protected by covering plates. Elevations pierced by double pores 

 ( = water-pores ?) are developed on more or fewer plates in many specie.s. 

 In others there are grooves pierced by pores which form a rhombic figure, 

 one part of the figure being on one plate, the other on an adjoining plate. 

 These pectinated rhombs are sometimes few in number, isolated, and 

 devoid of pores ; 'and the two halves of the rhomb are occasionally separate 

 and distinct. 



The Cystoidea appear in the Lower Silurian, reach a maximum of 

 development in the Upper Silurian, and die out in the Carboniferous strata 

 in which they occur sparingly. 



CLASS BLASTOIDEA. 



Extinct Pelmatozoa with a short stem, an ovate body, and five-rayed 

 ambulacral area. The calyx consists of three basals, five radials and five 

 (interradial) orals. Each radial is forked, inclosing a sinus, in which is 

 lodged an ambulacral area. The orals, or * deltoid pieces! are attached to 

 the ends of two adjoining prongs belonging to two different radials or 

 'fork pieces' The mouth is central. The ambulacral area is formed by 

 five lancet plates with median longitudinal grooves. Side plates rest upon 

 each lancet plate laterally, and in some forms there is also a row of outer 

 side plates. Jointed pinnules in a single or double row are attached to 

 the outer edge of each ambulacral groove. The inner side plates are 

 marked by grooves leading outwards, each one to a marginal pore. The 

 pores lead to a cleft (hydrospire cleft] at the outer edge of the lancet plate, 

 and the cleft in its turn to an underlying hydrospire canal, into which open 

 a system of interradial lamellar tubes, the hydrospires, supposed to be 



Pp 



