Sept. 2, 1880] 



NA TURE 



429 



fasciole can be followed. None of the genera of Petalosticha 

 belonging to the other groups develop any fasciole in the sense 

 of circumscribing a limited area of the test. 



The comparison of the genera of Echini w hich have appeared 

 since the Lias witli the young stages of growth of the principal 

 families of Echini, shows a most striking coincidence, amounting 

 almost to identity, between the successive fossil genera and the 

 X'arious stages of growth. This identity can, however, not be 

 traced exactly in the way in which it has usually been under- 

 stood, while there undoubtedly exists in the genera which have 

 appeared one after the other a gradual increase in certain families 

 in the number of forms, and a constant approach in each succeed- 

 ingformation, in the structure of the genera, to those of the present 

 day. It is only in the accordance bet« een some special points of 

 structure of these genera and the young stages of the Echini of the 

 present day that Me can trace an agreement which, as we go 

 further back in time, becomes more and more limited. We are 

 either compelled to seek for the origin of many structural features 

 in types of which we have no record, or else we must attempt to 

 find them existing potentially in groups where we had as yet not 

 succeeded in tracing them. The parallelism we have traced does 

 not extend to the structure as a whole. What we find is the 

 appearance among the fossil genera of certain structural features 

 giving to the particular stages we are comparing their charac- 

 teristic aspect. Thus, in the succession of the fossil genera, 

 when a structural feature has once made its appearance, it may 

 either remain as a persistent structure, or it may become gradually 

 modified in the succeeding genera of the same family, or it may 

 appear in another family, associated with other more marked 

 structural features which completely overshadow it. Take, for 

 instance, among the Desmosticha the modifications of the pori- 

 ferous zone of the actinal and abactinal systems of the coronal 

 plates, of the ambulacral and interanibulacral systems, the 

 changes in the relative proportion of the primary tubercles, and 

 the development of the secondaries. These are all structural 

 features which are modified independently one of the other ; we 

 may find simultaneous development of these features in parallel 

 lines, but a very different degree of development of any special 

 feature in separate families. 



This is as plainly shown in the embryological as in the palson- 

 tological development. In the Cidarida; there is the minimum 

 of specialisation in these structural features. In the Dia- 

 demopsida: there is a greater range in the diversity of the structure 

 of the poriferous zone and of the coronal plates, as well as of the 

 actinal system. There is a still greater range among the Echinidre, 

 while among the Salenida; the modifications, as compared to those 

 of the EchinidK and Diademopsidre, are somewhat limited 

 again, being restricted as far as relates to the poriferous zone 

 and coronal plates, but specialised as far as the actinal system 

 is concerned, and specially important ^^ith reference to the 

 structure of the apical system. The special lines in which these 

 modifications take place produce, of course, all possible com- 

 binations, yet they give us the key to the sudden appearance, as 

 it were, of structural features of whicli the relationship must be 

 sought in very distantly related groups. It is to this speciality 

 in the palaeontological development that we must trace, for 

 instance, the Cidarid affinities of the Salenire, their papilla;, 

 the existence of few large primary interanibulacral tubercles, 

 the structure of their apical system, and their large genital plates ; 

 while it is to their affinities with the Heniicidarida; that we must 

 refer the presence of the few larger primary ambulacral tubercles 

 at the base of the ambulacral area, and by their Diademopsid 

 and Echinidian affinities that we explain the indented imbricated 

 actinal system with the presence of a few genuine miliaries. 

 But all the structural features which characterise the earliest 

 types of the Desmosticha can in reality be traced, only in a 

 somewhat rudimentary form, even in the Cidaridie. The slight 

 undulation of the closely packed, nearly vertical poriferous zone 

 is the forerunner of the poriferous zone first separated into 

 vertical arcs and then into independent arcs. The limitation in 

 the number of the rows of granules in the ambulacral zone, and 

 their increase in size, is the first trace of the appearance of the 

 somewhat larger primary ambulacral tubercles of the Hemici- 

 daridce and Saleniaa. The existence of the smooth cylindrical 

 spines of the abactinal region of the test naturally leads to 

 similar spines covering the whole test in the other familie-; of the 

 Desmosticha. The difference existing in the plates covering the 

 actinal system from those of the coronal plates leads to the great 

 distinction between the structure of the actinal system and of 

 the coronal plates in some of the Echinidae. 



Passing to the Clypeastridse and Petalosticha, we trace a paral- 

 lelism of the same kind,and readily follow in the successive genera 

 of fossil Clypeastroids, but often in widely separated genera, the 

 precise modifications w hich the poriferous zone has undergone 

 as it first becomes known to us in Echinocyamus and Fibularia, 

 and as we find it in the most complicated petaloid stage of 

 the Clypeastroids of the present day. We readily trace the 

 changes the test undergoes from its comparatively ovoid and 

 swollen shape to assume first that of the less gibbous forms, 

 next that of the Laganida:, and finally of the flat Scutellida; ; 

 while we trace in the Echinanthida; the persistent structural 

 features of some of the earliest Clypeastroids, together with an 

 excessive modification of the poriferous zone. Likewise for the 

 EchinoconidK we trace mainly the slight modifications of the 

 poriferous zone and of the coronal plates, and finally, when we 

 come to the Spatangida; we find no difficulty in tracing from the 

 most Desmostichoid of the Spatangoid genera the modifications 

 of a test in wliich the ambulacral and interambulacral areas are 

 made up of plates of nearly uniform size, in which the anterior 

 and posterior extremities are barely specialised, to the most 

 typical of the Ananchytida?, in which the anterior and posterior 

 extremities liave developed the most opposite and extraordinary 

 structural features. In a similar way we can trace among the 

 fossil genera of different families the gradual development of the 

 actinal plastron from its very earliest appearance as a modifica- 

 tion of the posterior interambulacral area of the actinal side, or 

 the growtli of the posterior beak into an anal snout, the succes- 

 sive changes of the anal groove, the formation of the actinal 

 labium, or the development of the bourrelets and phyllodes from 

 a simple circular actinostome, the gradual deepening of the 

 slight anterior groove of some early Spatangoid to form the 

 deeply sunken actinal groove. Equally well we can trace the 

 tnodifications of the ambulacral system as it passes from the 

 simple poriferous zones of the earlier Spatangoids to genera in 

 which the petaliferous portion makes its appearance, and finally 

 becomes the specialised structure of our recent Spatangoid 

 genera, such as Schizaster, Moira, and the like. Finally, we 

 can trace to a certain extent the development of the fascioles on 

 one side from genera like Hemiaster, in which the peripetalous 

 fasciole is prominent, to genera like Brissopsis, Brissus, and the 

 like, of the present day ; on the other, perhaps, or both com- 

 bined, the foi-mation of a lateral and anal fasciole from genera 

 Uke Micraster in Spatangus and Agassizia. Thus we must, on 

 the same theory of the independent modifications of special 

 structural features, trace the many and complicated affinities 

 which so constantly strike us in making comparative studies, 

 and which render it impossible for us to express the manifold 

 affinities we notice, without taking up separately each special 

 structure. Any attempt to take up a combination of characters, 

 or a system of combinations, is sure to lead us to indefinite 

 problems far beyond our power to grasp. 



In the oldest fossil Clypeastroids and Petalosticha, as well as 

 in the Desmosticha, we also find the potential expression of the 

 greater number of the modifications subsequently carried out in 

 genera of later date. The semipetaloid structure of some of the 

 earlier genera of Spatangoids, the slight modifications of some 

 of the plates of the actinal side near the actinostome, are the 

 precursors, the one of the highly complicated petaloid ambulacra 

 of the recent Spatangoids, the other of the actinal plastron, 

 leading as it does also to the important differences subsequently 

 developed in tlie anterior and posterior extremities of the test, 

 as well as to the modifications which lead to the existence 

 of a highly labiate actinostome. The appearance of a ie\\ 

 miliaries near the actinostome constitutes the first rudimentary 

 bourrelets. 



Going back now to the Palsechinid^, the earliest representatives 

 of the Echini in pala:ozoic times, without any attempt to trace 

 the descent of any special type from them, we may perhaps find 

 some clue to the probable modifications of their principal struc- 

 tural features preparatory to their gradual disappearance. In 

 the structure of the coronal plates, the specialisation of the 

 actinal and abactinal systems, the conditions of the ambulacral 

 system, we must compare them to stages in the embryonic deve- 

 lopment of our recent Echini with which we find no analogues 

 in the fossil Echini of the Lias and the subsequent formations. 

 In order to make our parallelism, we must go back to a stage in 

 the embr}'onic history of the young Echini in which the distinction 

 to be made between the ambulacral and interambulacral systems 

 is very indefinite, in which the apical system is, it is true, special- 

 ised, but in whicli the actinal system remains practically a part 



