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



abactinal regions also exists iu these earliest Echinoidea, though it is so modified as to 

 show us how the existence of but a single peristomic system [Bothriocidaris) gradually 

 passes into the Palceechrnus stage, next into the Cidaris stage, and finally the Echinus 

 stage, and also how it is possible for the peculiar development of the test of the Petalos- 

 ticha even to be foreshadowed in the absence of lines of demarcation between these systems 

 in the Palseechinidse, and finally, how the existence of an anal system in one of the inter- 

 ambulacral areas, while calling to mind an eminently crinoidal structural feature, yet at 

 the same time shows the intimate relationship there may have existed between the 

 earliest Spatangoids and the immediate successors of the Cystechinidae, which thus also 

 may be the precursors of the excentric development of some of the Desmosticha, and of 

 the whole of the Clyi^eastroids and Spatangoids. 



If we examine in the same manner any one of the structural features which have 

 once made their appearance, we find that, without exception, they are either persistent to 

 the present day, or can be traced in a somewhat modified form in some one of the types 

 now living, though the peculiar combination of any definite numljer of these may have 

 disappear; d, and thus radically new elements may seem to have been introduced into 

 certain periods, which are after all only excessive modifications of a single element of 

 structure, which in other forms remains unmodified. This will explain, perhaps, more 

 vividly than any systematic descriptions of aflinities the subtle connections which close 

 examination of almost any genus of Sea-urchins at a special period shows not only to the 

 past but also to the future, and the endless links which can readily be traced by a 

 careful analysis between apparently totally disconnected types. Let us take as an 

 example one of the most recent genera, — the genus Sputangus, — and see how far back we 

 can trace the structural features, modified so as to be characteristic of Spatangus. 



The compact abactinal system we can, as I have already stated, trace to the encroach- 

 ment of the madreporite upon the difi"erent genital plates, and the gradual driving out of 

 the anal system into the odd interambulacral zone ; or we may go further back and trace 

 this asymmetric arrangement back to its crinoidal afiinities, this unequal development 

 of the difi'erent radial and interradial zones dating back to the earliest Echinodermal 

 structure, and being also naturally connected with the excentric position of the apical system 

 of the actinostome, and the elongation of the test. The strong contrast between the 

 actinal and abactinal surface goes back to the existence of the earliest Desmosticha. The 

 existence of large primary tubercles dates back to the Archseocidaridse, that of small primary 

 tubercles uniformly distributed over the test goes back to the Palseechinidje, the j)resence of 

 spines of two difi'erent kinds is as old as the earliest Sea-urchin, as well as the specialisation 

 of certain parts of the poriferous zone, and the existence of a specialised actinostome. 



The development of an actinal plastron dates back to the Galeritidge, to the first 

 disturbing element which the introduction of the anal system into the odd interam- 

 bulacral area brought in, and connected with that comes in the development of an anal 



