442 Prof. S. Lov^n on the Structure of the Echinoidea. 



probable that careful investigations will show that this inde- 

 pendent central disk may recur in the irregular Echinidse in 

 the median area of the vertex, which is penetrated by the 

 madreporite, and in many of them {Pygurus, Glyjpeus^ and 

 others) is very large. 



If we get rid of the notion that the vertical plates are an 

 appendage of the generative organs and the central disk of 

 the alimentary canal, if we see the latter in young Echinidge 

 in its original state, and consider, moreover, the vertex of the 

 Salenidte as being entire and not eroded from the periproctium, 

 we obtain an arrangement of these parts of the skeleton the 

 nearest homologue of which is to be sought in an Echinoderm 

 of a class which is regarded as very distinct from the Echi- 

 noidea. The Marsupites, only known as fossil, a Crinoid 

 without a peduncle, has in the pole opposite to the mouth a 

 single pentagonal disk closely embraced by the five basalia. 

 It is the same arrangement as that of the vertex in the young 

 of Echinus and in the Salenidas. The central disk and the 

 basalia, with the rest of the plates in Marsupites^ have strise 

 or grooves which are perpendicular to the suture, and may be 

 traced up to the middle of the plate, and make their appear- 

 ance most distinctly when its outermost calcareous layer is eaten 

 away. But this character is no peculiarity of Marsupites or 

 of the numerous Crinoidea in which it occurs. If we carefully 

 examine the central disk and vertical plates in small Echinidse, 

 we find exactly the same structure. It may be discerned even 

 on the sm-face by direct light, but is exceedingly distinct by 

 transmitted light and suitable treatment. We see the penta- 

 gonal plate divided into five triangular areas, which have its 

 five sides for their bases, their apices united in its middle ; the 

 reticulated texture is arranged, in each area, so that straight 

 parallel rods perpendicular to the base have narrow interspaces 

 between them. In the middle of the plate the rods and inter- 

 spaces of the difierent areas meet together and unite, crossing 

 each other in a closed and apparently irregular network ; but 

 in the sutures those of one plate are seen to meet those of 

 another in the same direction. This structure recurs in all 

 plates in the Echinoidea, whether regular or irregular, and is 

 the same that has long been known in the Cystidea. In the 

 fossil Salenidse these striae are seen very generally ; and the 

 structure of parallel rods in the interior is very distinct in a 

 living Salenia from the great depths near the Antilles, for 

 which, as for a Pygaster from the same depths, and numerous 

 other valuable objects, our Koyal Museum is indebted to 

 Dr. Axel Goes's conscientious and indefatigable researches. 



Another trait which expresses the homology between the 



