ON THE EMBRYOLOGY OF ECHINODERMS. 



epaulettes, v", fig. 16, have assumed a more independent position, forming a curv'e 

 somewhat similar to the arc from which the median anal arms of the Brachiolaria 

 are developed ; the third pair of arms bulges out quite prominently, c", when seen in 

 profile ; the fourth pair of arms is visible, e" ; the rod which eventually extends in the 

 interior of it is a straiglit rod (r") with a slight point in the middle, at present discon- 

 nected from the remaining part of the calcareous framework. This set of rods and the 

 fork r", which extends into the arms e'\ take their origin independently from the main 

 rod, extending from the anal part round by the mouth, from which branches are sent 

 into the arms e' and e'". The rod r" ultimately combines with the main system, but 

 the rod r'" always remains separate from the others. The position of these rods is 

 better understood from fig. 17 when seen from the aboral side. 



The stage represented in fig. 17 is particularly important, as it is at this time that 

 we notice the first trace of Avhat I suppose becomes the tentacular 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, of Plate VII. of Muller's seventh 

 Memoir,* where he figures a similar tentacular loop in two different stages of develop- 

 ment ; unfortunately, there is nothing in the text to explain what Miiller considered 

 it to be. The relation of this loop to the madreporic body is perfectly plain in this 

 same larva seen from above as it is floating in the water, fig. 18, the epaulettes 

 appearing like great flaps extending between the base of the arms e' and e", in con- 

 tinuation of the chord of cilia extending along these arms. 



Figs. 17 and 18 are larvae twenty-three days old ; during the next four days no 

 change of any importance could be perceived ; the tentacular loop remained the same, 

 the arms alone increasing in size, and a few dark pigment spots appeared in the arms. 

 Unfortunately, at the end of these four weeks the young Sea-urchins all died. I have 

 only once succeeded in keeping them such a long time, and that was during the coldest 

 winter weather. In the attempts made in the spring, whenever a warm day came, it 

 was sure to kill everything ; while in the summer, though the facilities I had were 

 infinitely greater, I never could keep these larva? alive more than three or four days. 

 The Sea-urchins spawn during the whole year. Successful artificial fecundations have 

 been made in December, January, and during every month from that time till the 

 middle of October. 



• MiJLLER, J. Ueber die Gattungen der Seeigellarven. Berlin, 1855. 



