Sept. 2, 1880] 



NATURE 



425 



Uie one side, and Batrachians and Fislies on the other. It is to 

 embryology that we owe the explanation of the affinities of the 

 Old Fishes in which Agassiz first recognised the similarity to the 

 embryo of Fishes now living, and by its aid we may hope to 

 understand the relationship of the oldest representatives of the 

 class. It has given us the only explanation of the early appear- 

 ance of the Cartilaginous Fishes, and of the probable formation 

 of the earliest vertebrate limb from the lateral embryonic fold, 

 still to be traced in the young of the Osseous Fishes of to-day. 



Embryology has helped us to understand the changes aquatic 

 animals must gradually undergo in order to become capable of 

 living upon dry land. It has given us pictures of swimming- 

 bladders existing as rudimentary lungs in Fishes with a branchial 

 system ; in Batrachians it has shown us the persistence of a 

 branchial system side by side with a veritable lung. We find 

 among the earliest terrestrial vertebrates types having manifest 

 affinities with the Fishes on one side and Batrachians on the other, 

 and we call these types Reptiles ; but we should nevertheless do 

 so with a reservation, looking to embryology for the true 

 meaning of these half-fledged Reptiles, which lived at the 

 period of transition between an aquatic and a terrestrial life, 

 and must therefore always retain an unusual importance in the 

 stiidy of the development of animal life. 



When we come to the embryology of the marine Invertebrates 

 the history of the development of the barnacles is too familiar to 

 be dwelt upjn, and I need only allude to the well-known trans- 

 formations of the Echinoderm-, of the Acalephs, Polyps, in fact 

 of every single class of Invertebrates, and perhaps in none more 

 than in the Brachiopods, to show how far-reaching has been the 

 influenc:; of embryology in guiding us to a correct reading of the 

 relations between the fossils of successive formations. There is 

 scarcely an embryological monograph now published dealing 

 with any of the later stages of growth which does not speak of 

 their re-emblance to some type of the group long ago extinct. 

 It has therefore been most natural to combine with the attempts 

 constantly made to establish the genetic sequence between the 

 genera of successive formations an effort to establish also a 

 correspondence between their pala.'ontological sequence and that 

 of the embryonic stages of development of the same, thus 

 extending the mere similarity first observed between certain 

 stages to a far broader generalisation. 



It would carry me too far to sketch out, except in a most 

 general way, even for a single class, the agreement known to 

 exist in certain groups between their embryonic development and 

 their palxontological history. It is hinted at in the succession 

 of animal life of any period we may take up, and perhaps cannot 

 be better expressed than by comparing the fauna of any period 

 as a whole with that of following epochs;— a zoological system 

 of the Jura, for instance, compared with one made up for the 

 Cretaceous; next, one for the Tertiary, compared with the fauna 

 of the present day. In no case could we find any class of the 

 animal kingdom bearing the same definitions or characterised in 

 the same manner. But apply to tliis comparison the data 

 obtained from the enibryological development of our present 

 fauna, and what a flood of light is thrown upon the meaning of 

 the succession of these apparently disconnected animal kingdoms, 

 belonging to difterent geological periods, especially in connection 

 with the study of the fesv ancient types \\hich have survived 

 to the present day from the earliest times in the history of our 

 earth ! 



Although there is hardly a class of the animal kingdom in 

 which some mo-t interesting parallelism could not be drawn, 

 and while the material for an examination of this parallelism is 

 partially available for the Fishes, Molluscs, Crustacea, Cor.als, and 

 Crinoids, yet for the illustration and critical examination of this 

 parallelism I have been led to choose to-day a very limited 

 group, that of Sea-urchins, both on account of the nature of the 

 material and of my own famiUari:y with their development and 

 with the living and extinct species of Echini. The number of 

 living species is not very great, — less than three hundred, — and 

 the number of fossil species thus far known is not, according to 

 Zittel, more than about two thousand. It is therefore possible 

 for a specialist to know of his own knowledge the greater part 

 of the species of the group. It has been my good fortune to 

 examine all but a few of the species now known to exist, and 

 the collections to which I have had access contain representatives 

 of the majority of the fossil species. Sea-urchins are found in 

 the oldest fossiliferous rocks ; they have continued to exist 

 without interruption in all the strata up to the present time. 

 While it is true that our knowledge of the Sea-urchins occurring 



before the Jurassic period is not very satisfactory, it is yet com- 

 plete enough for the purposes of the present essay, as it will 

 enable me, starting from the Jurassic period, to call your attention 

 to the pateontological history of the group, and to compare the 

 succession of its members with the embryological development 

 of the types now living in our seas. Ample material for making 

 this comparison is fortunately at hand ; it is material of a pecu- 

 liar kind, not easily obtained, and which thus far has not greatly 

 attracted the attention of zoologists. 



Interesting and important as are the earliest stages of embry- 

 onic development in the different classes of the animal kingdom, 

 as bearing upon the history of the first appearance of any organ 

 and its subsequent modifications, they throw but little light on 

 the subject before us. What we need for our comparisons are 

 the various stages of growth through which the young Sea- 

 urchins of different families pass from the time they have prac- 

 tically become Sea-urchins until they have attained the stage 

 which we now dignify with the name of species. Few embry- 

 ologists have carried their investigations into the more extended 

 field of the changes the embryo undergoes when it begins to be 

 recognised as belonging to a special class, and when the know- 

 ledge of the specialist is absolutely needed to trace the bearing 

 of the changes undergone, and to understand their full meaning. 

 Fortunately the growth of the young Echini has been traced in 

 a sufficient number of families to enable me to draw the paral- 

 lelism between these various stages of growth and the pala;onto- 

 logical stages in a very different manner from what is possible 

 in other groups of the animal kingdom, where we are over- 

 whelmed with the number of species, as in the Insects or 

 Mollusks, or where the pala;ontological or the embryological 

 terms of comparison are wanting or very imperfect. 



Beginning with the palosontological history of the regular sea- 

 urchins at the time of the Trias, when they constituted an unim- 

 portant group as compared with the Crinoids, we find the Echini 

 of that time limited to representatives of two families. One of 

 these, the genus Cidaris, has continued to exist, with slight 

 modifications, up to the present time, and not less than one-tenth 

 of all the known species of fossil Echini belong to this important 

 genus, which in our tropical seas is still a prominent one. It is 

 interesting here to note that in the Cidaridce the modifications of 

 the test are not striking, and the fossil genera appearing in the 

 successive formations are distinguished by characters which often 

 leave us in doubt as to the genus to which many species should 

 be referred. In the genus Rhabdocidaris, which appears in the 

 lower jura, and which is mainly characterised by the extraordi- 

 nary development of the radioles, we find the extreme of the 

 variations of the spines in this family. From that time to the 

 present day the most striking differences have existed in the 

 shape of the spines, not only of closely allied genera, but even 

 in specimens of the same species ; differences which in some of 

 the species of to-day are as great as in older geological periods. 

 The oldest Cidaridae are remarkable for their narrow poriferous 

 zones. It is only in the Jura that they widen somewhat ; sub- 

 sequently the pores become conjugated, and only later, during 

 the Cretaceous period, do we find the first traces of any orna- 

 mentation of the test (Temnocidaris) so marked at the present 

 day in the genus Goniocidaris. As far, then, as the Cidarida; 

 are concerned, the modifications which take place from their 

 earliest appearance are restricted to slight changes in the pori- 

 ferous zone and in the ornamentation of the test, accompanied 

 with great variability in the shape of the primary radioles. We 

 must except from this statement the genera Diplocidaris and 

 Tetracidaris, to which I shall refer again. Tlie representatives 

 of the other Triassic family become extinct in the lower tertiaries. 

 The oldest genus, Hemicidaris, undoubtedly represents the 

 earliest deviations from the true Cidaris type ; modifications 

 which affect not only the poriferous zone, but the test, the actinal 

 and the abactinal systems, while from tlie extent of these minor 

 changes we can trace out the gradual development of some of 

 the characteristics in families of the regular Echini now living. 

 The "enus Hemicidaris may be considered as a Cidaris in which 

 the poriferous zone is narrow and undulating, in which the 

 <Tanules of the ambulacral system have become minute tubercles 

 m the upper portion of the zone and small primary tubercles in 

 its actinal region, in which many of the inter-ambulacral granules 

 become small secondaries, in which the plates of the actinal 

 system have become reduced in number, and the apical system 

 has become a narrow ring, and finally, in which the primary 

 radioles no longer assume the fantastic shapes so common anion ; 

 the Cidaridae. 



