422 



THE CIRRIPEDS, TUNICATES AND ECHINODERMS. 



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 sub- 

 sequent metamorphosis, more than 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 organic 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 till 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 creat- 

 ing 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, like that which occurs so 

 frequently as an abnormality in Limulus, or even as a normal character in the 



FIG. 289. Cirriped larvae. E, Early nauplius stage, seen from the neural surface; F, from the side; G, metanauplius 



sage. Semi-diagrammatic. 



bopeiridae and paguridae, 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, 

 and of their resemblance to the tunicates, enteropneusta, and to the other chordate 

 phyla. 



The Echionderm Larva. A young echinoderm larva may be represented 

 in a generalized diagrammatic form as shown in Figs. 291, 293. In form and 

 structure it is much like the familiar cirriped nauplius, but differs from it in general 

 appearance, largely because it has no chitenous covering, and because it begins 

 its free swimming existence (the absence of yolk demanding an early liberation of 



