14 EMBRYOLOGY OF THE STARFISH. Part I. 



lines all round the body, and their only means of locomotion (PI II. v, v', Figs. 

 20-28). These rows are at first two very short arcs (v, v, PI. II. Fig. 22), with their 

 convexities placed opposite one another on each side of the depression in which the 

 mouth is placed (v, v, PI. II. Fig. 21). 



The general outline of the larva has, up to this stage (PI. II. Fig. 20), undergone 

 but slight modifications, the changes taking place principally in the digestive organs. 

 The phases through which the larva passes in the next three days, are of a 

 very different character ; the alimentary canal, the stomach, and the oesophagus, 

 become more circumscribed by the increasing difference there is noticed in the walls 

 of these regions. The stomach (d) is always marked by the greater thickness of 

 its walls; while, with increasing age, the walls of the oesophagus (o) become more 

 attenuated, and capable of greater expansion and contraction (PL II. Figs. 25, o, 27). 

 We notice, also, a rapid increase in the growth of the water-tubes (u>, ?//), which by 

 the end of the sixth day (PI. II. Figs. 27, 28) extend as far as the corners of the 

 mouth and along the edge of the walls of the stomach, towards the anal extremity 

 (PL II. Figs. 24, 26, w, iv). When viewed in profile (PL II. Figs. 25, 27), it will 

 be seen that the plane in which these water-tubes run, is not parallel to the longitu- 

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

 these two tubes. It is in these stages, represented in PL 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 moment on the changes which are 

 going on here, and compare them to what we find in other Radiates. Miiller has 

 always maintained, that the Echinoderm larvae being bilateral, we had a passage 

 from- a bilateral symmetry to a radiate type, while, in reality, this seeming bilat- 

 erality 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 

 larvae 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 fact, 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 gradualby becomes, by th»e 

 symmetrical arrangement of the edges of these plastrons, which leads one involun- 

 tarily 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 yet no one has attempted, for that reason, to make bilateral animals of the 

 Echinoderms. The Spatangoids might as well be called bilateral, and not radiating 



