126 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



neural side down, by means of larval appendages and a cephalic outgrowth; they 

 then turn neural side up and remain so attached for life; or in some cases they give 

 up their sessile existence and again become free, moving slowly about, neural side 

 down. There are, therefore, three chief characteristics of the echinoderms that 

 demand our first consideration: (1) The early bilateral symmetry and metamerism; 

 (2) the sessile life and mode of attachment by cephalic outgrowths; and (3) the 

 asymmetry. There appears to be but one explanation for these remarkable condi- 

 tions, which is as follows: The early development of bilateral sj-rnmetry and metam- 

 erism in the echinoderms, and the presence of a telocele and telopore in place of 

 the more primitive gastrula and blastopore, clearly indicate that they had their 

 origin in bilaterally symmetrical animals of the acraniate type that had already 

 acquired a considerable degree of complexity. These ancestral forms probably 

 belonged to the cirriped group, for before the latent asymmetry becomes effective 

 the young echinoderm larva resembles a cirriped in its form, mode of attachment, 

 and subsequent metamorphosis more than it does any other animal. The radiate 

 structure of the later stages was due to a persistent local defect, or to the absence 

 of a definite part of the embryonic formative material, which in turn created a con- 

 dition of unstable equilibrium, the result of which is that the whole side, following 

 the path of least resistance, bends toward the defective area, forming an arch that 

 increases in curvature until an approximate 'equilibrium is again attained by the 

 union of its two ends to form a circle. The original half metameres and segmental 

 organs are then arranged in radiating lines, thus creating a new radiate type and 

 a new set of internal conditions that dominate the future growth of the organism. 

 If we assume that a strongly marked asymmetry, such as that which occurs so fre- 

 quently as an abnormality in Xiphosura, or even as a normal character in the 

 Bopyridse and Paguridse, was a fixed feature of the hypothetical ancestral crrripeds 

 and was capable of a successful organic adjustment, we shall have a perfectly 

 simple and natural explanation of the origin and structure of the echinoderms." 

 "The young asteroid larva is said to attach itself voluntarily at first, and for a 

 short time only; later it becomes permanently attached, head first and neural side 

 down, in the same remarkable manner as a young cirriped, both the cephalic 

 appendages (which are thick walled and muscular, with a long basal portion and a 

 short terminal knob studded with small adhesive papillag, greatly resembling the 

 minute adhesive antennae of the cirripeds and parasitic crustaceans) and the adhesive 

 disk taking part in the process. The young crinoid larva attaches itself wholly 

 by means of the cephalic disk, as the adhesive appendages appear to be absent. 

 Its first position is with the neural or oral surface down, as in the cypress stage of 

 the cirriped. The disk then elongates, forming a slender cephalic stalk or peduncle, 

 and the larva turns a somersault, bringing its neural side uppermost. Meanwhile 

 the vestibule, or peribranchial chamber, which at first is small and temporarily closed, 

 enlarges, then ruptures, and 'the five appendages project from the cuplike 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 altogether and the young 

 echinoderm, after its metamorphosis, again acquires a limited power of locomotion. 



