HOMOLOGIES OF ECHIXODERMS. 93 



in the position of the water-tube in Echini and Ophiurans, the hxtter are 

 really more closely allied structurally to Starfishes than to Echini. This 

 will readily account for the position of the water-system inside of the test 

 in Ophiurans and Echini, contrasted to the Starfishes. AVe can also 

 homologize Holothurians with Echini by supposing that in that group 

 the limestone plates never form ambulacral and interambulacral plates, 

 but that the abactinal system of the embrj-o, as it elongates, covers 

 irregularly the water-system, the suckers of which pierce the plates as 

 they do in the embryonic stages of other Echinoderms. In fact, the 

 external limestone plates forming the test of a Sea-urchin, the reticulated 

 network of the actinal and abactinal surface of a Starfish together with 

 the ambulacral and interambulacral plates and the plates forming the disk 

 of an Ophiuran, the upper, lower, and side arm-plates, as well as internal 

 skeleton, are all directly derived from the simple system of limestone 

 plates of the abactinal surface of the Echinoderm embryo. This system 

 consists, in all cases, of a basal plate, five radial and five interradial 

 plates. In Ophiurans the genital plates are formed from the angles 

 of the five interradial plates ; similar plates can still be traced in the 

 young Starfishes, while in the full-grown Starfishes their presence is 

 shown by the interbrachial partition, on each side of which the ovaries 

 discharge. Thus there exists a complete homology between the genital 

 plates of Ophiurans and the interbrachial partitions of Starfishes, a ho- 

 mology fully carried out in its details when we examine the relations 

 held by the genital jjlates to the ovaries in Ophiurans and by the inter- 

 brachial partitions to the ovarian openings in Starfishes. 



From the primitive number of plates existing in the disks of all em- 

 bryo Echinoderms, it is evident that palaeontologists have laid altogether 

 too much stress upon the arrangement of the plates of the arms in 

 Crinoids. The study of the -solid parts of Starfishes, while valuable as 

 accessories, would certainly furnish no very satisfoctory data for a classi- 

 fication, at least if this were based entirely upon an examination of the 

 hard parts of the abactinal system alone, as is so frequently the case in 

 Crinoids. 



