758 HOMOLOGIES AND AFFINITIES OF ECHINI. 



There is a time, in the growth of all the Echinoderm embryos I have 

 examined, when, after the resorption of the pluteus, there is no difference in 

 the general structure of the young Ophiuran, Starfish, or Sea-Urchin. This 

 stage is represented by a condition in which there are neither ambulacral 

 nor interambulacral plates developed. — when the ambulacral svstem exists 

 on the actinal side as a simple tube from which the more or less rudimentary 

 suckers diverge as simple diverticula ; in this stage the young Echinoderm is 

 composed only of an actinal and of an abactinal side. The limestone net- 

 work, covering completely the whole of the abactinal surface, is not yet sub- 

 divided into plates, neither genital nor ocular plates are developed, and only 

 such coronal plates exist in Starfishes and Ophiurans as correspond to the 

 centre of the disk and to the base of the arms. The spines, as well as the 

 more or less indented outline, guide us in forming an idea of the order to 

 which the young embryo is to belong. The anal opening is an aperture 

 which in sea-urchins early reaches its ultimate position. The teeth of Echini 

 also early appear, totally disconnected from the test. In the earlier stages 

 of growth, even after the suckers have become encased in the limestone net- 

 work of the actinal surface, the plates which are to form the ambulacral and 

 interambulacral plates are not differentiated, and are only formed subse- 

 quently from what remains always (in Starfishes and Ophiurans) the abac- 

 tinal part of thi' body, but which in Echini forms the so-called coronal plates, 

 — modifications only of the primitive abactinal system of starfishes, as far as 

 the plates immediately adjoining the ambulacral tubes are concerned. 



This introduces some changes into the generally received homologies of 



Echinoderms, and gives us. perhaps more satisfactorily than any other expla- 

 nation, the homologies of Ophiurans. which, having no interambulacral sys- 

 tem, have remained, more than the starfishes, in the condition most closely 

 approaching that of the typical Echinoderm embryo after its resorption of 

 the pluteus; that is. the plates formed along the arms are not differentiated 

 into the systems recognized in Starfishes and Echini, and remain always a part 

 of the original primitive abactinal system, subsequently to the pluteus stage. 

 We may also explain more satisfactorily the homologies of Holothurians, 

 which would thus, in such forms as Synapta, where the ambulacral suckers are 

 limited to those immediately round the actinostome, remind us of the most 

 embryonic conditions which we find in Echini, Starfishes, and Ophiurans. 

 The primitive abactinal system becomes greatly elongated, the ambulacral 

 tubes extend with it to the other extremity, and no special ambulacral or 



