Sept. 2, i8So] 



NATURE 



427 



affinity, and perhaps even to a direct and not very distant rela- 

 tionsliip with the Palrechinidas. For if we analyse tlie Echino- 

 thuria: of the present day, we find in genera like Phormosoma 

 many structural features, such as the shape of tlie test, the 

 character of the spines, the stracture of the apical system, that 

 of the poriferous zone, indicative of possible modifications in 

 the direction of Pygaster or of Galeropygus, which have as yet 

 not been taken ijito account. 



Adopting for the Petalosticha the same method of tracing the 

 modifications of single structiu-al features in their pateontological 

 succession, we trace the comparatively little modified pala;onto- 

 logical history of the Echinoneidn: of the present day from the 

 Pyrina of the lower Jura. This, in its turn, has been preceded 

 by Hyboclypus and Galeropygus, while the Echinolampads of 

 the present day date back, vtkh. but trifling modifications, to the 

 Echinobrissus of the Lias, itself preceded by Clypeus : and they 

 have been subject only to slight generic changes since that time, 

 Echinobrissus being still extant, while such closely-allied genera 

 as Catopygus and Cassidulus of the earlier Cretaceous are still 

 represented at the present day ; the modifications taking place 

 in the actinal system, in the ambulacral zones of the Echino- 

 conidaa and of the Echinolampadje showing the closest possible 

 systematic affinity in these families. Starting again from Hybo- 

 clypus, with its elongate apical system, we naturally pass to 

 Collyrites and the strange Dysasteridce forms which in their turn 

 are closely allied to the Holasteridre. From Holaster on the 

 one side and from Toxaster on the other, we find an unbroken 

 sequence of structural characters uniting the successive genera of 

 Holasteridre, such as Cardiaster, Offaster, Stenonia, Ananchytes, 

 and Asterostoma, with Paleopneustes, Homolampas, and the 

 Pom-talesix of the present day, while from the genera of the 

 Toxasteridce we naturally pass to the cretaceous Hemiaster ; in 

 this genus and the subsequent Micraster we find all the elements 

 necessary for the modifications which appear in the Spatanginje 

 from the time of the Chalk to the present day. These modifi- 

 cations result in genera in which we trace the development of 

 the fascioles, of the actinal, anal, and abactinal plastrons, of the 

 beak, the formation of the petaloid ambulacra, first flush with 

 the test, and little by little changed into marsupial pouches, the 

 growth of the anterior groove and the manifold modifications of 

 the ambulacral system in Spatangus, Agassizia, and Echinocar- 

 dium, often recalling in some of its features structural characters 

 of families which have preceded this in time. 



Apparently in striking contrast with the Echini of the secondary 

 period and those which have succeeded them stand the PaL-eozoic 

 Echini ; but when we have examined the embryology of Echini, 

 we shall be better prepared to understand their structure and the 

 afiinities of the Patechinida; with the Echini of the present day, 

 and their immediate predecessors. 



Taking up now the embryological development of the several 

 families which will form the basis of our comparisons, beginning 

 with the Cidaridse, we find that in the earliest stages they very 

 soon assume the characters of the adult, the changes being 

 limited to the development of the abactinal system, the increase 

 in number of the coronal plates, and the modifications of the 

 proportionally gigantic primary radioles. 



In the Diadematida: the changes undergone by the younc are 

 limited to the gi-adual transformation of the embryonic spines to 

 those which characterise the family, to the changes of the 

 vertical row of pores in the ambulacral area into arcs°of three or 

 four pairs of pores, and to the specialisation of the actinal and 

 abactinal systems. 



In the Arbaciadc-e the young stages are remarkable for the 

 prominent sculpture of the lest, for the flattened spines, for their 

 simple poriferous zone, for their actinal system, and for their 

 genital ring. The anal plates appear before the genital ring. 



In the Echinometradse the " young thus far observed are 

 characterised by the small number of their primary tubercles, 

 the large size of the spines, the simple vertical row of pores, the 

 closing of the anal ring by a single plate, aud the turban-shaped 

 outline of the test Little by little the test loses \vith increasing 

 age this Cidaris-like character ; it reminds us, from the increase 

 in the number of its plates, more of Hemicidaris ; then, with 

 their still greater increase, of the Pseudodiadematidx ; and, 

 finally, of the Echinometradx proper. The spines, following 

 /«;•( /assii the changes of the test, lose little by Utile their 

 fantastic embryonic, or rather Cidaris-like appearance, and 

 become more solid and shorter, tiU they finally assume the 

 delicately fluted structure characteristic of the Echinometrada\ 

 The vertical poriferous zone is first changed into a series of 



connected vertical arcs, which become disjointed, and form, with 

 increasing age, the independent arcs of pores, of three or more 

 pairs of pores, of the Echinometrada?. 



In the Echinidae proper we find in the young stages the same 

 unbroken vertical line of pores, which graduallybecomeschan-^ed 

 to the characteristic generic types. AVe find, as in the Echino- 

 metrada;, an anal system closed with a single plate, and an 

 abactinal system separating in somewhat more advanced stages 

 from the coronal plates of the test. This is as yet made up of 

 a comparatively small number of plates, carryin'^ but few laro-e 

 irimary tubercles, with fantastically shaped spin'es entirely out 

 of proportion to the test, but which, little by little, with the 

 increa^^e of the number of coronal plates, the addition of piimary 

 tubercles, and their proportional decrease in size, assume more 

 and more the structure of the genus to which the young belono-s. 

 The original anal plate is gradually lost sight of from the increase 

 in number of the plates covering the anal system, and it is only 

 among the Temnopleurida: that this anal plate remains more or 

 less prominent in the adult. In the Salenida;, of which we know- 

 as yet nothing of the development, this embryonic plate remains 

 permanently a prominent structural feature of the apical system. 1 



Among the Clypeastroids the changes of form they undergo 

 during growth are most instructive. We have in the young 

 Fibularina: an ovoid test, a small number of coronal plates 

 surmounted by few and large primary tubercles, supporting 

 proportionally equally large primary radioles, simple rectilinear 

 poriferous zones, no petaloid ambulacra— in fact, scarcely one of 

 the features we are accustomed to associate with the Clypeastroids 

 is as yet prominently developed. But rapidly, with increasing 

 size, the number of primary tubercles increases, the spines lose 

 their disproportionate size, the pores of the abactinal region 

 become crowded and elongate, and a rudimentary petal is formed. 

 The test becomes more flattened, the coronal plates increase in 

 number, and it would be impossible to recognise in the young 

 Echinocyamus, for instance, the adult of the Cidaris-like or 

 Echinometra-like stages of the Sea-urchin, had we not traced 

 them step by step. Most interesting, also, is it to follow the 

 migrations of the anal system, which, to a certain extent, may 

 be said to retain the embryonic features of the early stages of all 

 Echinoderm embryos, in being placed in more or less close 

 proximity to the actinostome. What ha=; taken place in the 

 growth of the young Echinocyamus is practically repeated for all 

 the families of Clypeastroids ; a young Echinarachnius, or 

 Mellita, or Encope, or a Clypeaster proper, resembles at first 

 more an Echinometra than a Clypeastroid ; they all have simple 

 poriferous zones and spines and tubercles out of all proportion 

 to the size of the test.^ 



When we come to the development of the Spatangoids, we 

 find their younger stages also differing greatly from the adult. 

 Among the Nucleolida:, for instance, the young stages have as 

 yet no petals, but only simple rectilinear poriferous zones. They 

 are elliptical with a high test, with a single large primary tubercle 

 for each plate, and a simple elliptical actinostome, without any 

 trace of the typical bourrelets and phyllodes so characteristic of 

 this family. Very early, however, this condition of things is 

 changed, the test soon becomes more flattened, the petals begin 

 to form as they do in the Clypeastroids, and we can soon trace 

 the rudiments of the peculiar bourrelets characteristic of the 

 family, accompanied by a rapid increase in the number of 

 tubercles and in that of the coronal plates. 



Among the Spatangida; some are remarkable in their adult 

 condition for their labiate actinostome, for the great develop- 

 ment of the petals, for the presence of fascioles sun'ounding cer- 

 tain definite are.is, for the small size of the tubercles, the general 

 uniformity in the spines of the test, and the specialisation of 

 their anterior and posterior regions. On examining the young 

 stages of this group of Spatangoids, not one of these stiaictural 

 features is as yet developed. The actinostome is simple, the 

 poriferous zone has the same simple structure from the actino- 

 stome to the apex, the primary tubercles are large, few in 

 number, surrounded by spines which would more readily pass as 

 the spines of Cidarida; than of Spatangoids. The fascioles [are 

 either very indistinctly indicated, or else the special lines have 



' The young of the following genera have served as a basis for the pre- 

 ceding analysis of the embryonic stages of the Desmoslicha : Cidarls, 

 Dorocidaris, Goniocidaris, Arbacia, Porocidaris, Strongylocentrotus, Echi 

 nometra, Echinus, Toxopneustes, Hipponoe, Temnopleurus. Temnechinus, 

 and Trigonocidaris. 



* Among the Clj-peastroids I have examined the young of Echinocyamus, 

 Fibalaria, Meilita, Laganum, Echinarachnius, Encope, Clypeaster, and 

 Echinanthus. 



