SCIENCE. 



147 



their affinities with the Hemicidaridae that we must refer 

 the presence of the few larger primary ambulacral tubercles 

 at the base of the ambulacral area, and by their Diademop- 

 sid 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 

 characterize the earliest types of the Desmosticha can in 

 reality be traced, only in a somewhat rudimentary form, 

 even in the Cidaridas. 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 Hemi- 

 cidaridae and Saleniae. The existence of the smooth cylin- 

 drical spines of the abactinal region of the test naturally 

 leads to similar spines covering the whole test in the other 

 families in 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 Clypeastridae and Petalosticha, we trace a 

 parallelism of the same kind, and readily in the successive 

 genera of fossil Clypeastroids, but often in widely separated 

 genera, the precise modifications which the poriferous zone 

 has undergone as it first becomes known to us in Echi- 

 nocyamus and Fibularia, and as we find it in the most com- 

 plicated petaloid stage of the Clypeastroids of thepresent 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 Laganidae, and 

 finally of the flat Scutellidae ; while we trace in the Echinan- 

 thidae the persistent structural features of some of the ear- 

 liest Clypeastroids, together with an excessive modification 

 of the poriferous zone. Likewise for the Echinoconidae we 

 trace mainly the slight modifications of the poriferous zone 

 and of the coronal plates, and finally, when we come to the 

 Spatangidae we find no difficulty in tracing from the most 

 Desmostichoid of the Spatangoid genera, the modifications 

 of a test in which the ambulacral and interambulacral areas 

 are made up of plates of nearly uniform s'ze, in which the 

 anterior and posterior extremities are barely specialized, to 

 the most typical of the Ananchytidae, in which the anterior 

 and posterior extremities have 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 modification of the posterior inter- 

 ambulacral area of the actinal side, or the growth of the 

 posterior beak into an anal snout, the successive 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 modifications 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 ap- 

 pearance, and finally becomes the specialized 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 in both combined, 

 the formation of a lateral and anal fasciole from genera like 

 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 separ- 

 ately each special structure. Any attempt to take up a com- 

 bination 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 Demosticha, we also find the potential ex- 

 pression of the greater number of the modifications subse- 



quently carried out in genera of later date. The semipeta- 

 loid 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 

 Spantangoids, the other of the actinal plastron, leading as 

 it does also to the important differences subsequently de- 

 veloped in the anterior and posterior extremities of the 

 test, as well as to the modifications which lead to the exist- 

 ence of a highly labiate actinostome. The appearance of a 

 few miliaries near the actinostome constitutes the first ru- 

 dimentary bourrelets. 



Going back now to the Palaechinidae, the earliest repre- 

 sentatives of the Echini in paleozoic times, without any 

 attempt to trace the descent of any special type from them, 

 we may perhaps find some clew to the probable modifica- 

 tions of their principal structural features preparatory to 

 their gradual disappearance. In the structure of the coro- 

 nal plates, the specialization of the actinal and abactinal 

 systems, the conditions of the ambulacral system, we must 

 compare them to stages in the embryonic development 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 embryonic history of the young Echini in which 

 the distinction to be made between the ambulacral and in- 

 terambulacral systems is very indefinite, in which the apical 

 system is, it is true, specialized, but in which the actinal 

 system remains practically a part of the coronal system. 

 But here the comparison ceases, and, although we can 

 trace in the paleontological development of such types as 

 Archaeocidaris or Bothriocidaris modifications which would 

 lead us without great difficulty, on the one side to the Cida- 

 ridae. and on the other to the Echinothuriae and Diadema- 

 tidae of the present day, we cannot fail to see most definite 

 indications in some of the structural features of the Palae- 

 chinidae of characteristics which we have been accustomed 

 to associate with higher groups. The minute tubercula- 

 tion, for instance, of the Clypeastroids and Spantangoids, 

 already existing in the Melonitidae, the genital ring, and 

 anal system, are quite as much Echinid as Cidarid. The 

 polyporous genera of the group represent to a certain ex- 

 tent the polypori of the regular Echini, and the lapping of 

 the actinal plates of the Cidaridae and of the coronal plates 

 in some of the Diadematidae, as well as the existence of 

 such genera as Tetracidaris, of four interambulacral plates 

 in Astropyga, and of a large number of ambulacral plates in 

 some of the recent Echinometradse, all these are Palaechinid 

 characters which we can explain on the theory of the inde- 

 pendent development of the structural features of which 

 they are modifications. We should, hewever, remember, 

 that the existence of a large number of coronal plates, espe- 

 cially interambulacral plates, in the Palaechinidae, is a mere 

 vegetative character, which they hold in common with all 

 the Crinoids — a character which is reduced to a minimum 

 among the Holothurians, and still persists in full force 

 among the Pentacrini of the present day, as well as the 

 Astrophytidte and Echinidae. 



It would lead me too far to institute the same comparison 

 between the embryonic stages of the different orders of 

 Echinoderms and their earliest fossil representatives. We 

 may, however, in a very general way, state that we know 

 the earliest embryonic stages of the orders of Echinoderms 

 of to-day, which, with the exception of the Blastoidea and 

 Cystideans, are identical with the fossil orders, and that as 

 far as we know they all begin at a stage where it would be 

 impossible to distinguish a Sea-urchin from a Star-fish, or 

 an Ophiuran, or a Crinoid, or an Holothurian — a stage in 

 which the test, calyx, abactinal and ambulacral systems are 

 reduced to a minimum. From this identical origin there is 

 developed at the present day, in a comparatively short 

 period of time, either a Star-fish, a Sea-urchin, or a Cri- 

 noid ; and if we have been able successfully to compare, in 

 the development of typical structures, the embryonic stages 

 of the young Echini with their development in the fossil 

 genera, we may fairly assume that the same process is ap- 

 plicable when instituting the comparison within the differ- 

 ent limits of the orders, but with the same restrictions. 

 That is, if we wish to form some idea of the probable course 

 of transformations which the earliest Echinoderms have 

 undergone to lead us to those of the present day, we are 



