470 Mr. O. H. Grosvenor. [Nov. 3, 



la cause, elle est difficile a pene'trer. On sait que les ^Eolidiens 

 se nourrissent frequemment de Ccelenteres (Hydraires, Actinies), et 

 paraissent doues d'immunite vis-a-vis de leur nematocystes ; or il est 

 curieux de se remarquer que c'est precise'ment une espece qui ne 

 s'attaque pas a des Coelenteres, qui est depourvue de sacs cnidophores." 



We see from this case that the degeneration and atrophy of the 

 cnidosacs follows very soon the abandonment of a Coelenterate diet. 



Fiona is another genus unprovided with cnidosacs. I have not been 

 able to find any record of the food of this genus, but Dr. Lo Bianco 

 tells me that about 3 years ago large numbers of Fiona nobilis, Velella 

 and Janthina, were taken together in the Bay of Naples, and that he 

 observed the Fionse feeding on Velella, the individuals which had so 

 fed being coloured a deep blue. Velella and Fiona were also taken 

 this spring in company, and I observed Fiona devouring the siphono- 

 phore ; a normally coloured Fiona becomes blue very soon after feeding 

 on Velella and its gastric diverticula become crowded with nemato- 

 cysts. The blue colour persists for a considerable time (a week to 

 10 days) without a further supply of Velella, so that if this was the 

 staple food of this species, we should expect the majority to be blue, 

 which is not the case. Besides those taken with Velella, a number of 

 Fiona nobilis were found on floating pieces of pumice or wood covered 

 with a species of Lepas. I kept one such colony in a large jar of 

 sea-water, and next morning the bodies of half-a-dozen barnacles were 

 lying on the bottom, their stalks having disappeared. As the remaining 

 barnacles were quite healthy, I think there can be no doubt that the 

 stalks had been devoured by the Nudibranchs. It seems probable, there- 

 fore, that the food of Fiona is varied, and that the proportion of 

 Ccelenterates is not sufficient to give a constant supply of nematocysts, 

 so that the cnidosacs have been lost or never developed. 



5. The inadequacy of the published accounts of the development of 

 nematocysts in ^Eolids is remarked on by Glaser, who also points out 

 that even if undoubted developmental stages were found, it would not 

 prove that they arose in the JEolid, for they might well be introduced 

 together with the fully formed ones. The comparative scarcity of 

 young stages (I myself have never found anything which I could 

 confidently affirm to be such) I take to be due to the following 

 causes : 



(1) The development of nematocysts in the less accessible basal part 



of the polyp and their migration only when fully formed to the 

 exposed positions where they are of use. 



(2) The greater digestibility of the young stages. Spirocysts must 



also be digestible, for though they are as numerous as 

 nematocysts in many Actinians, they are not found in actinian- 

 eating jEolids. But Bedot states that he has found them in 

 Pleurophyllidia (3). 



