6 ON THE EMBRYOLOGY OF ECHINODERMS. 



which are already more familiar to us from the drawings of Miiller ; they resemble 

 closely some of the figures given by him of Toxopneustes lividus in his fourth Memoir. 

 A Sfood deal of allowance must be made for the diflferences of outline between the 

 figures given here and the drawings of Miiller. From the evidence of the drawings 

 themselves, it is plain that nearly all the specimens drawn by him are compressed. 

 I have endeavored to represent these larvae as they appear swimming about ; it is by 

 no means an easy task to follow them in their almost unceasing movements with the 

 magnifying power required to introduce the necessary details, but I trust I have suc- 

 ceeded in giving a tolerably accurate idea of their appearance in these outline draw- 

 ings. In a larva during the tenth day after fecundation (fig. 14), the most important 

 changes are the increase of the arms e', e"', and the formation of rudiments of another 

 pair of arms, e" ; the vibratile epaulettes, v", as Miiller has called the peculiar accumu- 

 lation of vibratile cilia situated between the base of the adjoining arms e', e'", make 

 their appearance at this stage. It is easy to follow them from their origin, when they 

 are simply a thickening of the vibratile chord v", figs. 14, 15, until they have passed 

 through the successive stages represented in figs. 16, 17, 19, to attain the great size 

 observed in fig. 20, when they appear in certain positions as having no connection 

 whatever with the vibratile chord, and to have originated independently of the main 

 chord. Miiller had not traced their development, and laid great stress on their pres- 

 ence in distinguishing the diff"erent species of Sea-urchin larvae. 



With the development of the arms, the intestine loses its fomier shape ; it has now 

 assumed the appearance of a large elliptical receiver with thin walls. The stomach is 

 somewhat dumb-bell-shaped, and the left water-tube connects with the surrounding 

 water, through the water-pore h, having pushed its way to the surface. The rods keep 

 pace with the growth of the arms, fig. 15 ; the water- tubes have not increased in size ; 

 they are still two distinct bodies. The anal part of the outline of the larva is quite 

 pointed ; the aboral side is regularly arched, with a slight depression at the point 

 where the water-pore opens, h, fig. 14. The opacity of the larva has increased to such 

 an extent that it becomes impossible to define clearly the outline of the water-tubes 

 in the stages which come between figs. 14 and 16. I am unable to state positively 

 whether the two water-tubes are united in this and older larva;. All I could distinctly 

 see was the great increase in size of the water-tubes ; but at the same time it becomes 

 a puzzling matter to trace the limits of these tubes, owing to the delicate walls which 

 bound them. Their presence can only be traced by the fine line which runs across the 

 oesophagus from each side, and by the water-pore and the tube leading to it; h, fig. 16. 

 In a profile view of a larva considerably older than that represented in fig. 15, the 



