On tlie Structiire and Affinities of Mnestra parasites Krolm etc. 51 



It is true that some parts of the Phyllirhoe are within the 

 ränge of a few of the thrcad-cells and that their discharge might 

 have some stimulative effect upon tlie host, but the large exumbral 

 hatteries of the Mnestra can never Ije l)rought to bear upon tlic 

 PhiflUrhoe. Consequently, if these exumbral batteries serve any 

 useful purpose, as their great efficiencv indicates that they do, 

 that purpose must be either offensive or defensive against other 

 organisms than the Phyllirhoe] but on the other haud, owing to the 

 Position and sessility of the Mnestra, the batteries can onlv be 

 brought iuto action with the couuivance of the Phyllirhoe. 



It is easier to suggest than to prove that the Phyllirhoe acts 

 as if it were aware of the lethal weapon it carries on its neck. 

 On one occasion only have I witnessed an action which would lend 

 colour to this Suggestion. The PhyUirhoe was slowly swimming 

 along the bottom of a glass aquarium, and Perseus-like was holding 

 its Mediisa-head displayed. A young fish encountered the pair. It 

 swam quietly aloug the side of the Phyllirhoe, but then on Coming 

 in contact with the finestra, immediately recoiled as if it had 

 touched a dangerous object. However such an isolatcd Observation 

 as this cannot be acccpted as more than a possible indication of the 

 protection a Mnestra might be to the Phyllirhoe. 



To sum np: — ■ 



Mnestra makes use of Phyllirhoe for the nutriment cer- 

 tainly of itself, and possibly of its germ cells and young 

 embryoes. 



Phyllirhoe may make use oi Mnestra as a means of de- 

 fence against enemies. 



In consequence, the association is to be regarded as one of 

 symbiosis rather than of parasitism and that therefore the specific 

 uame of Mnestra parasites does not give an entire indication of its 

 relation to its host. 



Affinities and System atic Position. 



In spite of the fact that we are still not quite certain about 

 the place of development of the genital cells in Mnestra, yet we 

 are sufficiently well acquainted with other characteristics of taxo- 

 nomic value to assert definitely that it should be eurolled among 



4* 



