THE KCHIXODERM LARVA. 



423 



the embryo) with the aid of cilia instead of appendages. But in both larvae there 

 is the same enormous labrum, /.;the same caudal lobe, al., with its similarly placed 

 anus; the same lateral thoracic folds enclosing a central depressed area; a similarly 

 located adhesive disc, and the same simple, U-shaped, alimentary canal. 



The ciliated band is one of the characteristic features of the echinoderm larva. 

 It has been compared with the prototroch of annelids and molluscs, but it is of an 

 entirely different nature. Its main course is longitudinal, and when completed it 

 surrounds the neural surface only (Fig. 293, //.) on the other hand, the proto- 

 troch is always equatorial, extending around the long axis, across both neural and 

 haemal surfaces. (Fig. 267, pt.) 



a. 



ab 



a. 



FIG. 290. FIG. 291. 



FIG. 290. A hypothetical cirriped-like larva, in which the posterior part of the trunk is taking on a false 

 radial symmetry due to the absence of the left half of the middle g^oup of metameres. The figure is designed to 

 illustrate the probable origin of radial symmetry in the echinoderms. 



FIG. 291. Diagram of a late stage in the development of a star-fish larva. The asymmetrical metanauplius 

 stage, before the asymmetry has produced the characteristic radial structure of the later stages. 



The ciliated band of the echinoderm larva is merely an adaptation, or modi- 

 fication, of the thickened and sensory margins of the thoracic folds and of the 

 preoral and caudal lobes. In embryonic arthropods, the margins of the thoracic 

 folds, and to a less degree those of the preoral and caudal lobes, are studded with 

 rows of minute hairs, many of which are sensory. Long before the folds are actu- 

 ally formed, or any chitenous covering is secreted, their future location is clearly 

 indicated by a deeply stained, thickened band of ectoderm. In Limulus the band is 

 first formed as tw r o lateral thickenings, which extend forw r ard and backward, 

 unite, and form a continuous girdle around the neural surface. (Figs. 140-142.) 



When the echinoderm larva grows older, the ciliated band is thrown into folds, 

 or tentacle-like lobes (Fig. 292), the larger ones corresponding, approximately, 

 to the marginal outgrowths so characteristic of the cirripeds. (Figs. 289 and 290.) 



