718 



EMBRYOLOGY. 



the fork r". which extends into the arms e", take their origin independently 

 of the main rod which extends from the anal part round the mouth, and from 

 which branches are sent into the arms e and e". The rod r" ultimately com- 

 bines with the main system, but the rod r" always remains separate from the 

 others. The position of these rods is better understood when seen from the 

 aboral side {Fig. ~, .')■ 



The stage represented in Fig. 52 is particularly important, as it is at this 

 time that we notice the first trace of what I suppose becomes the tentacular 



Fir.. 53. 



pentagon of the young sea-urchin. On the left water-tube we notice a very 

 prominent loop t, which, from its resemblance to the tentacular loops of 

 Brachiolaria. and from its position on the water-tube connecting with the 

 water-pore, I have no hesitation in considering to be the first tentacular 

 loop formed. Compare Figs. 1, 2. Plate MI. of Midler's seventh Memoir* 

 where he figures a similar tentacular loop in two different stages of devel- 

 opment; unfortunately there is nothing in the text to explain what Mliller 

 considered it to be. The relation of the loop to the madreporic body is per- 

 fectly plain in this same pluteus seen from above as it floats in the water 

 (Fig. .;.;)• the epaulettes appearing like great flaps extending between the 



» Muli.kr, J.. Ueber die Gattungen der Seeigellarven. Berlin, 1855. 



