and Enibryological Development. 367 



The appearance of a few miliaries near the actinostome con- 

 stitutes the first rudimentary bourrelets. 



Going back now to the PalsechinicUe, the earliest repre- 

 sentatives of the Echini in Palaeozoic 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 structural features preparatory to their 

 gradual disappearance. In the structure of the coronal 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 interambulacral systems is 

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

 cialized, 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 palseontological develop- 

 ment of such types as Archceocidaris or Bothriocidaris modi- 

 fications which would lead us, without great difficulty, on the 

 one side to the Cidaridae, and on the other to the Echino- 

 thurice and Diadematidee of the present day, we cannot fail 

 to see most definite indications in some of the structural 

 features of the Palaechinidai of characteristics which we have 

 been accustomed to associate with higher groups. The 

 minute tuberculation, for instance, of the Clypeastroids and 

 Spatangoids, 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 

 extent, the Polypori of the regular Echini ; and the lapping of 

 the actinal plates of the Cidaridse 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 Echinometradas — all these are Paltechinid cha- 

 racters which we can explain on the theory of the independent 

 development of the structural features of which they are modi- 

 fications. We should, however, remember that the existence 

 of a large number of coronal plates, especially interambulacral 

 plates, in the Pakechinidae 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 Astrophytidge and Echinidse. 



