MONOGRAPH OF THE EXISTING CEINOIDS. 7 



the Echiaoidea, and the Holothuroidea) and the astroradiatc (including the 

 Asteroidea and the Ophiuroidea) between which there are, and can be, no interme- 

 diates. 



Thus it is evident that we must use the very greatest care in the correlation of 

 the chronogeny and the phylogeny of the echinoderms, and we must be continually 

 on the watch for sudden and aberrant deviations and specializations in the older as 

 well as in the more recent types. A detailed study of the hving types will furnish 

 the key to many such deviations, and this subsequently will enable us correctly to 

 interpret the comphcated morphology of the extinct species. 



As nearly as I can see there is comparatively httle of value to be learned in the 

 first instance from the palseontological record of the echinoderms, at least in so far 

 as their comparative morphology and phylogeny is concerned, which can not be 

 learned just as well, or even better, from a study of the recent forms alone, though 

 the fossils furnish invaluable confirmatory evidence of the truth of any conclusions 

 which we may reach. 



If we acquire our facts from a study of the comparative anatomy, morphology 

 and development of the recent types and then test them by reference to the extinct 

 series, it seems to me that we can build up eventually a logical phylogenetic sequence 

 of types of progressively increasing speciahzation and perfection which will be able 

 to withstand all the attacks which may be made upon it. 



Of the many and varied recent forms there is abundant material, and this 

 material is always susceptible of detailed study. Furthermore, all of the recent 

 types are interconnected by readily demonstrable phylogenetic lines with all the 

 others. 



On the other hand, among the fossils reaUj' good and satisfactor}^ specimens are 

 rare, and there are many interesting forms which we are not able, on palseontological 

 evidence alone, to connect in a truly satisfactory manner with related types. 



In treating of the interrelationships of the various echinoderm groups it -will 

 be noticed that I have not taken the larvte into consideration. The larvse of the 

 echinoderms are very highly specialized creatures, specialized for a mode of life 

 enthely different from that of the adults, and hence specialized in an entirely different 

 way. To aU intents and purposes they are organisms of a different class entirely. 

 Moreover, they are not all specialized in the same direction, and hence are not 

 strictly comparable among themselves. Mechanical considerations of form make 

 comparison between the barrel-shaped larva of Aniedon, the bipinnaria of Asterias, 

 the auricularia of Holothuria, and the plutei of OpJiiura or of Echinus hazardous 

 and unsatisfactory. 



A true comparison between the species of the several echinoderm groups is only 

 possible upon the attainment of the adult form, or at the earliest at the inception 

 of the pentamerous symmetry. However suggestive and instructive the larvse may 

 be, they must be treated quite separately from the adults, as a distinct class of ani- 

 mals, or trouble is sure to result. 



In this respect I consider the echinoderms as a whole precisely comparable to 

 those msects and crustaceans which undergo a complete metamorphosis, though in 



