1903.] On the Nematocysts of Solids. 467 



ship,* for nothing short of the polyphyletic origin of JEolids from diverse 

 Coelenterates would bring us any nearer an explanation. 



2. Nematocysts of different types occurring in individuals of the 

 same species, or in the same individual. 



The following instances will illustrate this : 



i. Of 48 R. peregrina, while all had small pyriform nematocysts 



(fig. i), 



(a) 20 had no others, 



(b) 16 had also large bean-shaped nematocysts, like those of 



Eudendrium (fig. 4), 



(c) 10 had both these, and also small round Tubularia-like 



nematocysts (fig. 3), 



(d) 1 had only small pyriform and small round nematocysts, 



(e) 1 all the forms found in (c), and in addition a few similar 



to those shown in fig. 6, but about half the size. 



ii. Of 29 specimens of Spurilla neapolitana, 26 (including 4 young 

 specimens from 1*5 2 cm. long) had nematocysts shown in 

 fig. 8 ; three very young specimens (less than 1 cm. long.) had 

 nematocysts like those in fig. 9. This may, perhaps, indicate 

 that a change of food is a regular occurrence in the life-history 

 of this species. 



iii. Of two young specimens of Facelina punctata, one had nemato- 

 cysts of at least five different kinds, shown in figs. 1, 3 and 5 ; 

 the other had nematocysts of two kinds indistinguishable from 

 those of Pennaria Cavolinii (see below), (fig. 2). 



Such individual variations have often been noticed by writers on 

 this group, and in his recent paper on the development of the cerata, 

 Krembzow remarks on such a case in Aeolidiella glauca, one of his 

 specimens having a different type of nematocysts from the others ; he 

 seems almost inclined to consider it a different species on this account, 

 though after a careful examination he was unable to discover any 

 other difference, anatomical or histological. 



I imagine that what uniformity in this respect does exist (and this 

 must be considerable in the majority of species, or the idea of the 

 development of nematocysts within the Solids would never have 

 endured so long), must be due to a decided preference on the part of 

 each species of ^Eolid for a particular kind of hydroid, for living and 

 feeding on which they are specially adapted. In this connection the 

 case of Flabellina a/finis and Coryphella Lansburgii is interesting. These 

 species are by no means nearly related, being placed by Bergh in 

 separate sub-families. But in external appearance they show a 

 remarkable resemblance. The specimens I have examined show 

 * Vide Lankester's ' Treatise of Zoology,' Part 1Y, p. 12. 



