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 Toocopneustes lividus in his fourth Memoir. 

 A good deal of allowance 



must be made for the differences of 



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 



t> 



f the arms 



e 



in 



and the formation of rudiments of 



pair of arms 



,iv 



the vibratile epaulettes, v", as Miiller has called the peculiar accumu 



lation of vibratile cilia situated between 



their appearance at this stag 



base of the adjoining arms e\ e"' 



m 



ake 



It 



easy to follow them from their origin, when they 



are simply a thickening of the vibratile chord v'\ figs. 14, 15 



through the successive 



g 



represented in figs. 16, 17, 19 



they have passed 

 in the great size 



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



hatever with the vibratile chord, and 



hav 



chord. Miiller had not traced the 



mated independently of the main 



development, and laid great stress on their pres- 



ence in distinguishing the different species of Sea-urchin larvae. 



With the development of the arms, the intestine loses its former shape 

 assumed the appearance of a large elliptical receiver with 

 somewhat dumb-bell-shaped, and the left water 



The stomach 



tube connects with the 



ound 



through the water-pore fc, having pushed its way to the surface 



o 



The rods keep 



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

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

 pointed ; the aboral side is re£ 



where the water-pore opens, &, fig. 14 



73. is quite 

 ularly arched, with a slight depression at the point 



an 



extent that it becomes impossible to define cl< 

 the stages which- come between figs. 14 and 16 



The opacity of the larva has increased to such 



ly the outline of the water-tubes 

 I am unable to state positively 



All I could distinctly 



whether the two water-tubes are united in this and older larvae 



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 tnbes, 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; b, fig. 16. 

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



/ 



