Chap. I. DEVELOPMENT OF THE PLASTRONS. 15 



animals, on account of the perfectly regular symmetrical arrangement of the fascioles, 

 upon the whole of the spheromeres of which the body of one of these Spatan- 

 goids is composed, and • in Avhich even the ambulacra! system presents marked 

 features 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. We should be as little 

 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 anterior and posterior one. In the embryo of our Starfish, which told so- 

 plainly, in its early stages, of the plan upon which it is built, that plan is now 

 lost sight of in the extraordinary bilateral development of some of the parts. 

 But, until Spatangoids and flat-soled Holothurians are proved to be truly bilateral 

 animals, and not genuine Radiates, with subordinate bilateral features, these seeming 

 bilateral Echinoderm larvae must be considered as truly radiate, with bilateral 

 features engrafted upon them. 



Development of the Plastrons. — The cylindrical shape, characterizing the earlier 

 stages of the larva, disappears soon after the* appearance of the first trace of the 

 appendages, which give to these larvae such a peculiar appearance, and they now 

 assume the features of the adult. The depression (PL II. Figs. 25, 27, m), in which 

 the mouth is placed, becomes more marked; we have a greater separation of the 

 oral (V) and anal (v) swellings of the vibratile chord, little by little changed into 

 two independent breastplates, the edges bound with chords of powerful vibratile 

 cilia, becoming the locomotive organs of the larvae (PL II. Figs. 20, 22, 24, 26, 28). 

 These plastrons, at first mere crescent-shaped shields (PL II. Figs. 20, 22, 24), extend 

 gradually towards either extremity, become elliptical, and then somewhat triangular. 

 The outline of the anal shield becomes sinuous, slight indentations point out the 

 position of the future arms (PL II. Fig. 26, e e, Fig. 28, e e, e'" e") ; the rows of cilia 

 creep gradually round the edge of this anal shield, turn towards the mouth again, 

 and extend, on the dorsal side, along the whole length of the larva (PL II. Fig. 25) ; 

 this chord of cilia makes a complete circuit, while the cilia, extending along the 

 edge of the oral plastron, do not meet. 



The formation of these plastrons is attended with great changes in the gen- 

 eral outline of the larva ; the anal extremity becomes pointed, triangular, with 

 rounded edges ; the body, on each side of the oral opening, bulges out beyond 

 the general outline, and the oral plastron is more and more pointed, as it sepa- 

 rates from the rest of the larva. This change of shape can perhaps be better 

 appreciated when seen in profile, and comparing the drawings of larvae three days 



