530 On Arachnactis brachiolata. 



group of greater value than a family, a suborder in the 

 order of Actinaria. 



In what way the additional tentacles of the edge of the 

 disk of Cerianthus are developed, I am unable to say, nor 

 can I state whether the tentacles which are first formed 

 always retain their greater size, as is the case in Arach- 

 nactis. 



The most advanced specimen of Arachnactis which I 

 have figured (fig. 2), has evidently not reached its full size, 

 as the sphere of yolk cells has not entirely disappeared, and 

 no sexual organs have been observed. 



EXPLANATION OF THE FIGURES. 



The figures are all placed with the tentacles down, as 

 that is the natural attitude of these animals while swim- 

 ming. 



Fig. 1. A young Arachnactis having four tentacles seen 

 from the narrow side, facing the pair of youngest tenta- 

 cles. The two small tentacles seen in the figure, are the 

 large pair of tentacles of the opposite extremity of the 

 mouth slit. 



Fig. 2. An Arachnactis brachiolata with thirteen margi- 

 nal tentacles also seen from the narrow side, facing the 

 pair of last formed tentacles, the digitate appendages are 

 seen symmetrically arranged on both sides of the column ; 

 the sphere of yolk cells is much reduced in proportion to 

 the size of the cavity. 



Fig. 3. The disk of a young Arachnactis seen from 

 above ; there are three pairs of marginal tentacles, the odd 

 tentacle is developed, and placed on the right of the 

 figure, the fourth pair of tentacles is still united in one 

 protuberance. The tentacles round the mouth slit are as 

 yet simj)le thickening of the walls round it projecting 

 straight up as seen in figures 1 and 5. 



Fig. 4. The same as figmre 2, seen from the narrow side 



