NO. II ECHINODERMS AS ABERRANT ARTHROPODS CLARK I9 



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: (i) The early 

 bilateral symmetry and metamerism; (2) the sessile life and mode 

 of attachment b}^ 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 sym- 

 metry and metamerism 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 prob- 

 ably 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 condition of unstable equilibrium, the result of which is that 

 the whole side, following the path of least resistence, bends toward 

 the defective area, forming an arch that increases in curvature until an 

 approximate equilibrium is again attained by the union of the 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 frequently as an abnormality 

 in Xiphosura, or even as a normal character in the Bopyridse and 

 Paguridse, was a fixed feature of the hypothetical ancestral cirripeds 

 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 papilhe, greatly resembling the minute 

 adhesive antennfe of the cirripeds and parasitic crustaceans) and the 

 adhesive disc taking part in the process. The young crinoid larva 

 attaches itself wholly by means of the cephalic disc, as the adhesive 



