20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



appendages appear to be absent. Its first position is with the neural 

 or oral surface down, as in the cypris stage of the cirriped. The disc 

 then elongates, forming a slender cephalic stalk or peduncle, and the 

 larva turns a somersault, bringing its neural side uppermost. Mean- 

 while the vestibule, or peribranchial chamber, which at first is small 

 and temporarily closed, enlarges, then ruptures, and the five ap- 

 pendages project from the cup-like head in typical cirriped fashion. 

 In certain of the representatives of the recent echinoderms, such as 

 the asteroids, the fixed stage is temporary, while in certain others, 

 such as the echinoids and holothurians, it appears to be omitted alto- 

 gether and the young echinoderm, after its metamorphosis, again 

 acquires a limited power of locomotion. But in most primitive echino- 

 derms, such as the stalked crinoids, blastoids and cystideans, a perma- 

 nent 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 does 

 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 structure as to the fact that 

 its freedom was gained at a late period in the phylogeny of a very 

 ancient group in which sessile inaction was the prevailing condition. 

 It is often assumed that a sessile or parasitic mode of life is the initial 

 cause of degeneration. The various anatomical peculiarities common 

 to the copepods, cirripeds and acraniates do not bear out this con- 

 clusion. The fact that in these diverse subphyla we see the same shift- 

 ing of cephalic appendages to the haemal side, the same cephalic out- 

 growths, and the same degeneration of the neuro-muscular organs, 

 indicates that there are certain initial defects or peculiarities of 

 germinal material common to the whole group, and that these are the 

 underlying cause of defective organization, the defective organization 

 being in every case of such a nature that a sessile or parasitic or 

 vegetative mode of life is the only one possible." 



