APPEAEANCE OF CILIAEY CHOEDS. 17 



plane in which these water-tubes run is not parallel to the longitudinal 

 axis, but inclined to it in such a manner, that the oesophagus passes 

 between these two tubes. It is in these stages, represented in PI. II. 

 Figs. 20-28, that the passage from the initial, truly radiate form to a 

 bilateral one is the most obvious, and it may be well to dwell for a mo- 

 ment on the changes which are going on here, and compare them to what 

 we find in other Radiates. Miiller has always maintained that, the Echi- 

 noderm larvie being bilateral, we had a passage from a bilateral symmetry 

 to a radiate type, while in reality this seeming bilaterality is subordinate 

 to a truly radiate plan of structure. The first question to settle with 

 regard to this is, whether we have a strictly bilateral form among the 

 larvje or not, and whether we do not find here a repetition of what is so 

 constantly met with in the animal kingdom, — the undue preponderance 

 of some parts, hiding effectually the plan upon which the whole animal 

 is built; in fxct, the engrafting of a subordinate type upon the type which 

 remains predominant. With the gradual development of the plastrons 

 alluded to, as formed from the chord of vibratile cilia, the embryo assumes 

 more and more a shape which renders it quite difficult to perceive the 

 original plan of radiation, concealed, as it gradually becomes, by the sym- 

 metrical arrangement of the edges of these plastrons, which leads one 

 involuntarily to mistake their mode of execution for the plan upon which 

 the animal is built. This apparent passage from a strictly radiating form 

 to a seeming bilateral one is nothing more than what we find constantly 

 among the adults of this same class, and j-et no one has attempted, for 

 that reason, to make bilateral animals of the Echinoderms. The Spatan- 

 goids might as well be called bilateral, and not radiating animals, on 

 account of the perfectly regular symmetrical arrangement of the fascioles, 

 extending over all the spheromeres composing the body of such Sjiatan- 

 goids, and in which even the ambulacral system presents marked fea- 

 tures of bilateral symmetry. The case is exactly a parallel one ; this 

 chord of vibratile cilia, and the chord of fascioles, arranged so regularly, 

 simply conceals in both cases the plan upon which the animal is built, 

 but does not, in either case, change the plan of radiation into that of 

 bilaterality. As little should we be justified in removing some of the 

 Holothurians, such as Cuviera and the like, from the Radiates, simply 

 because the greater preponderance of some of the ambulacra has brought 

 out, in these animals conspicuously, a dorsal and a ventral side, and an 



