428 



NATURE 



\_Sept. 2, 185^0 



not as yet made their appearance ; the ambulacral suckers of the 

 anterior zone are as large and prominent as those of the young 

 stages of any of the regular Echini. It is only little by little, 

 with advancing oge, that we begin to see signs of the specialisa- 

 tion of the anterior and posterior parts of the test, that we tind 

 the characteristic anal or lateral fascicles making their ajipear- 

 ance, only with increasing size that the spines lose their Cidaris- 

 like appearance, that the petals begin to be formed, and that the 

 simple actinostome develops a prominent posterior life. In the 

 genus Ilemiaster, the young stages are specially interesti]ig, as 

 long before the appearance of the petals, while the poriferous 

 zone is still simple, the total separation of the bivium and of the 

 trivium of the ambulacral system, so characteristic of the earliest 

 Spatangoids (the Dysasterince), is very apparent.' 



From this rapid sketch of the changes of growth in the prin- 

 cipal families of the recent Echini we can now indicate the 

 transformations of a more general character through which the 

 groups as a whole pass. 



In the first place, while still in the Pluteus all the young 

 Echini are remarkable for the small number of Coronal plates, 

 •for the absence of any separation between the actinal and abac- 

 tinal systems and the test proper. They all further agree in the 

 large size of the primary spines of the test, whether it be the 

 young of a Cidaris, an Arbacia, an Echinus, a Clypeaster, or a 

 Spatangoid. They all in their youngest stages have ^imple 

 vertical ambulacral zones ; beyond this, we find as chauges 

 characteristic of some of the Desmosticha, the specialisation of the 

 actinal system from the coronal plates, the formation of an anal 

 system, the rapid increase in the number of the coronal jlates, 

 with a corresponding increase in the number of the spines and a 

 proportional reduction of their size, the formation of an abac- 

 tinal ring, and the change of the simple vertical poriferous zone 

 into one composed of independent arcs. 



In the Spatangoids and Clypeastroids we find common to 

 both groups the shifting of the anal system to its definite jilace, 

 the modifications of the abactinal part of the simple ambulacral 

 system in order to become petaloid, and the gradual change of 

 the elliptical ovoid test of the young to the characteristic generic 

 test, accompanied by the rapid increase in the number of the 

 primary tubercles and spines. Finally limited to the Spatan- 

 goids are the changes they undergo in the transformation of the 

 simple actinostome to a labiate one, the specialisation of the 

 anterior and posterior parts of the test, and the definite formation 

 of the fasoioles. 



Comparing this embryonic development with the palajontolo- 

 gical one, we find a remarkable similarity in both, and in a 

 general way there seems to be a parallelism in the appearance of 

 the fossil genera and the successive stages of the development of 

 the Echini as we have traced it. 



We find that the earlier regular Echini all have more or less a 

 Cidaris-like look — that is, they are Echini with few coronal 

 plates, large primary tubercles, with radir.les of a corresponding 

 size ; that it is only somewhat later that tlie Diademopsidre make 

 their appearance, which, in their turn, c< rrespond within certain 

 limits to the modifications we have traced in the growth of the 

 young \ liadematidse and Arbaciada:. The separation of the 

 actinal system from the coronal plates has been effected. The 

 poriferous zone has either become undulating, or forms somewhat 

 indefinite open arcs ; we find in all the genera of this group a 

 larger number of coronal plates, more numerous primaries, the 

 granules of the Cidarida; replaced by secondaries and miliaries, 

 and traces of a Ilemicidaris-like stage in the size of the actinal 

 ambulacral tubercles. 



Comparing in the same way the palocontological development 

 of the Echinida: proper, we find that, on the whole, they agree 

 well with the changes of growth we can still follow to-day in 

 their representatives, and that, as we approach nearer the present 

 epoch, the fossil genera more and more assume the structural 

 features which we find developed last among the Echinid:e of 

 the present day. Very much in the same manner as a young 

 Echinus develops, they lose, little by little, first their Cidaridian 

 affinities, which become more and more indefinite, next their 

 Diadematidian affinities, if I may so call the young stages to 

 which they are most closely allied, and finally, with the increase 

 in the number of the coronal plates, the gi'eat numerical de\e- 

 lopment of the primary tubercles and spines, and that of the 

 secondaries and miliaries which we can trace in the fossil Echini 



* For this sketch of the embryology of the Petalosticha I have exnmined 

 theyoungof Echinolampas, Echinoneus, Echinocardium, Brissopsis, Agas^izia. 

 Spatangus, Brissus, and Hemiaster. 



of the Tertiaries, we pass insensibly into the generic types 

 characteristic of the present day. 



Although we know nothing of the embryology of the Salenidce, 

 yet, like the Cidarid.i?, they have in a great measure remained a 

 persistent type, the modifications of the group being all in the 

 same direction as those noticed in the other Desmosticha ; a 

 greater number of coronal plates, the development of secondaries 

 and miliaries combined with a specialisation of the actinal system 

 not found in the Cidaridte. 



An examination of the succession of the Echinoconida; shows 

 but little modification from the earliest types ; the changes, how- 

 ever, are similar to those undergone by the Clypeastroids and 

 Petalosticha, though they do not extend to modifications of the 

 poriferous zone, but are mainly changes in the actinostome and 

 in the tuberculation. In fact, the group of Echinoconida;' seems 

 to hold somewhat the same relation to the Clypeastroids which 

 the .Salenida; hold to the Cidarida;, and the earliest genus of 

 the group Pygaster has remained, like Cidaris, a persistent type 

 to the present day. 



The earliest Clypeastroids are all forms which resemble the 

 Fibularina and the genera following Echinocyamus and Fibu- 

 laria ; they are mainly characterised by the same changes which 

 an Echinarachnius or a Mellita, for instance, undergoes as it 

 passes from its Echinocyamus stage to the Laganum or En- 

 cope stage. The comparison is somewhat more complicated 

 when we come to the Spatangoids. The comparison of the 

 succession of genera in the different families, as traced in the 

 Desmosticha and Clypeastroids, is made difficult from the per- 

 sistency of the types preceding the Echinoneidse and the Anan- 

 chytidK, which have remained without important modifications 

 from the time of the lower Cretaceous ; previous to that time the 

 modifications of the Cassidulidce are found to agree with the 

 changes which have been observed in the growth of Echino- 

 lampas. The early genera, like Pygums, have many of the 

 characteristics of the test of the young Echinolampas. The de- 

 velopment of prominent bourrelets and of the floscelle and 

 petals goes on side by side with that of genera in which the 

 modification of the actinostome, of the test, and of the petals is 

 far less rapid, one group retaining the Echinoneus features, the 

 other culminating in the Echinolampas of the present day, and 

 having likewise a persistent type, Echinobrissus, which has re- 

 mained \^ith its main structural features unchanged from the 

 Jura to the present day. That is, we find genera of the Cassi- 

 dulida; which recall the early Echinoneus stage of Echinolampas, 

 next the Caratomus stage, after which the floscelle, lourrelets, 

 and petals of the group become more prominent features of the 

 succeeding genera. Accompanying the persistent type Echino- 

 brissus, genera appear in which either the bourrelets or petals 

 have undergone modifications more extensive than those of the 

 same parts in the genera of the Echinoneus or Caratomus type. 



The earliest Spatangoids belong to the Dysasterid.T?, apparently 

 an aberrant group, but which, from the history of the young 

 Hemiaster, we now know to be a strictly embryonic type, w hich, 

 while it thus has affinities with the true Spatangoids, still retains 

 features of the Cassidulida:- in the mode of development of the 

 actinostome and of the .petals, as well as of the anal system. 

 The genera following this group, Holaster and Toxaster, can 

 be well compared, the one to the young stages of Spatangus 

 proper before the appearance of the petals, w-hen the ambulacra 

 are flush with the test, and when its test is more or less ovoid, 

 the other to a somewhat more advanced stage, when the petals 

 have made their appearance as semi-petals. In both cases the 

 actinostome has the simple structure characteristic of all the 

 young Spatangoids. The changes we notice in the genera 

 which follow them lead in the one case through very slight 

 modifications of the abactinal system, of the anterior and 

 posterior extremities of the test, to the Ananchytid-like Spatan- 

 goids of the present day, the Pourtalesia', the genus Holaster 

 itself persisting till well into the middle of the Tertiary period ; 

 while on the other side we readily recognise in the Spatanginje 

 which follow Toxaster (a persistent type which has continued as 

 Palaiostoma to the present day) the genera which correspond to 

 the young stages of such Spatangoids as Spatangus and Brissopsis 

 of the present day, genera which, on the one hand, lead from 

 Hemiaster (itself still represented in the ]5resent epoch), through 

 stages such as Cyclaster, Peripneustes, Brissus, and Schizaster, 

 and, on the other, through Micraster and the like, to the Spatan- 

 goids, in which the development of the anal plastron and fascicle 

 performs an important part, while in the former group the 

 development of the peripetalous fasciolc and of the lateral 



