12 THE VOYAGE OF H.M.S. CHALLENGER. 



a change of which we can follow the traces in successive geological periods. The 

 position of this opening is a very variable one, and in the life history of a young 

 Sea-urchin it passes from its original place near the actinostome to one within the 

 apical system, plainly showing that during the earliest stages of growth the position 

 of the external opening of the extremity of the alimentary canal is still undetermined, 

 and that we must not attempt to find in the geological sequence any explanation of 

 this transfer any more than the position of the genital opening within or without the 

 genital plates gives us in the history of the growth of the Clypeastroids any clue to its 

 causes. 



The position of the anal opening among the Spatangoids is in reality, as has been shown 

 by Loven, not an advance, but the retaining of a structural feature once uniform among 

 the earliest Echinoderms, and which we find in all the Palseozoic Crinoids, but which may 

 nevertheless have very gradually been developed again during the geological succession, as 

 there are indications already in the Palaeechinidae of such an excentric position of the anal 

 system. And we find, as has been so well shown by Loven, in the older Echinoidea a 

 marked encroachment of the anal system upon one of the genital plates, which culminates 

 in some species of Acrosalenia ; and we may consider this as the last trace, perhaps, in the 

 regular Echinids of the excentric position of the anal system without the apical system, 

 the last trace of a condition of things which was more universal and which tends gradually 

 to be constituted as we find it in the Echinids of the present day. While in the groups 

 in which we find a retrogression, as it were, to the ancient condition of things we find it 

 accompanied by a renewal of the functions of the genital plates, and at the same time by 

 the encroachment of the madreporic body upon the other plates, thus often occupying the 

 whole central part of the apical system, and thus again giving us an explanation of the 

 many genital plates which may be occupied by parts of the madreporite in the earliest 

 Palgeechinidse, as we find it in several of the genera figured by Bailey, Worthen, and others. 

 And we can gradually trace both in the Echinoneidse and in the Cassidulidse the regular 

 geological succession existing between the genera wath the anal opening close to the apical 

 system, and those in which at the present day it is found on the actinal surface, and we see 

 that while the circular or globular species are the more ancient, they are little by little 

 replaced by species in which the longitudinal axis becomes more marked, the anal system at 

 the same time gradually passing from the apex towards the ambitus and finally to the actinal 

 side. This forms a striking contrast to the embryological fact that in young Clypeastroids 

 the anal opening is at first always on the abactinal side and gradually finds its way to the 

 actinal surface ; which does not seem to accord well with the view that this tranposition 

 of the anal opening is of a retrogressive character. 



The changes the apical system goes through the moment it has become independent 

 of the anal system are very important, and are connected also with other modifications in 

 the plates of the test which radically aff"ect its whole appearance. 



