472 Mr. G. H. Grosvenor. [Nov. 3, 



Rizzolia peregrina and Spurilla neapolitana, which are the species most 

 easily obtained in the late autumn at Naples. 



1. Four specimens of Rizzolia peregrina were obtained at the beginning 

 of November, being dredged with some tufts of Eudendrium. These 

 had nematocysts of one kind only, which were small (average length 

 6*5 /A), pyriform or pip-shaped, and are shown in fig. 1. These 

 specimens were placed in a small aquarium with several colonies of 

 Pennaria Cavolinii. The nematocysts of this hydroid are of two kinds, 

 which differ chiefly in size, the larger being about 25 //, the smaller 7 /A, 

 in length. They are oval, and slightly pointed at the end of the 

 capsule opposite the base of thread (fig. 2). It will be seen that these 

 could not possibly be confused with the nematocysts found in the 

 cnidosacs of the ^Eolids at the time of capture. 



For some days the Nudibranchs were too much occupied in deposit- 

 ing eggs to pay any attention to the unaccustomed food provided for 

 them, but on the ninth morning I noticed that several branches of 

 the hydroid were denuded of polyps, and, on examining the cerata of 

 the Solids, I found a considerable number of Pennaria nematocysts 

 mixed with the original ones, which were still preponderant in 

 numbers. 



Remembering Bergh's opinion, that one kind of nematocyst is 

 developed in the cnidosac, while others may be derived from hydroids, 

 I continued to feed these individuals on P. Cavolinii for a month. At 

 the end of this time the original nematocysts were almost entirely 

 replaced by those of Pennaria ; in some papillae I could not find any 

 but the latter, and in none that I examined did the small pyriform 

 cnidse amount to more than 2 per cent, of the total number in the 

 cnidosac. 



2. Four other specimens of R. peregrina were kept in an aquarium 

 with Eudendrium, Pennaria, Tubularia, and Aiptasia variabilis. These 

 specimens had at capture nematocysts of two kinds found in some 

 species of Eudendrium ; the small pyriform kind is shown in fig. 1 

 (occurring on the tentacles of the hydroid), and large bean-shaped ones, 

 shown in fig. 3 (found in a definite zone on the body of the polyp). 

 After a month in the aquarium they had, of course, still a number of 

 Eudendrium nematocysts, since this hydroid was included in their food ; 

 but they had also a considerable number of Pennaria and Tubularia 

 nematocysts, and a few from the acontia of Aiptasia, though the 

 latter is very different from their accustomed prey, and, in fact, 

 R. peregrina does not seem adapted to attack Actinians (see below). 

 Besides these there were a few oval nematocysts, with a considerable 

 length at the base of the thread, covered with fine barbs, which must 

 have been derived from a small hydroid inadvertently introduced with 

 the others, and which I have not been able to identify ; they certainly 

 cannot be the nematocysts developed in R. peregrina, since they only 



