128 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



But in most primitive echinoderms, such as the stalked cimoids, blastoids, and 

 cystideans, a permanent attachment by an elongated cephalic stalk, in typical 

 cirriped fashion, was the almost invariable rule, and no doubt represented the 

 primitive condition for the whole class. When an echinoderm docs become free 

 it acquires only a very limited power of locomotion and of coordinated movement. 

 Its characteristic lack of efficiency in this respect is due not so much to its simple 

 or primitive structure as to the fact that its freedom was gained at a lato period in 

 the phylogeny of a very ancient group in which sessile inaction was the prevailing 

 condition. It is often assumed that a sessUe or parasitic mode of life is the initial 

 cause of degeneration. The various anatomical peculiarities common to the cope- 

 pods, cirripeds, and acraniates do not bear out this conclusion. The fact that in 

 these diverse subphyla we see the same shifting of cephalic appendages to the 

 hfemal side, the same cephalic outgrowths, and the same degeneration of the neu- 

 romuscular organs, indicates that there 

 are certain initiixl defects or peculiarities 

 of germinal material common to the whole 

 group, and that these are the underl^ving 

 cause of defective organization, the defec- 

 tive organization being in every case of 

 such a nature that a sessile or parasitic or 

 vegetative mode of life is the only one 

 possible." 



Professor Patten doubts very much 

 whether it will ever be possible to make 

 precise or detailed comparisons of any 

 value between relatively modern types of 

 arthropods, like the decapods and insects, 

 and the echinoderms. My attention was 

 directed toward a comparison of the adults 

 of the two groups on account of the liigh 

 degree of specialization of the echinoderm 

 lai-vsc, and the difficulty of bringing into satisfactory correlation the data offered 

 by the very diverse young of the different echinoderm classes. 



Wliile it certainly is not possible to indicate any such close agreement between 

 the adults of crustaceans and echinoderms as has been shown by Prof. Patten to 

 exist in the case of the young, it appears to me that a description of an ecliinoderm 

 in terms of a crustacean, and a description of a crinoid in terms of other ecliinodcnns, 

 in the manner in which I originally worked them out, will prove to be not without 

 interest. 



The points of correspondence between the adult crustaceans and the adult echi- 

 noderms as indicated in the following pages are only to a very limited degree capable 

 of logical and connected proof as true homologies; collectively they form the base 

 for the construction of a working hj-pothesis through the adoption of which very 

 many problems in the comparative morphology of the echinoderms are logically 



Fig. 75.— a spehmex cu- Heteeometra rev.vaudii 



TROit CEVLON only partially CALCIFIED; (a) THE 



ENTIRE ANIMAL, AND (6) A SINGLE ARM FROM THE 

 SAME INDIVIDUAL. 



