LINNEAN SOCIETY OV LOIfDON. 59 



The small soft-shelled pea-crab, Pinnotlieres, is only found 

 sheltered in the mantle-cavity of some Lamellib ranch, to •^hich 

 the ancients tLouglit it acted as a sort o£ watch-dog, by adminis- 

 tering a nip to give warning of approaching danger. 



The genera Pontonia and Jypton, again, are only found similarly 

 protected in the interior of some animal. Of the latter genus, 

 Jypton spongicola is only found concealed in the cavity of a 

 sponge, which it closely resembles in colour and degree of trans- 

 parency, and has a curious habit o£ making a sharp noise by 

 snapping its chel», apparently with the object of frightening 

 away its enemies. 



Amongst the LameUibranchiata, Montacuta is found on the 

 spines of Spatangus, gaining 2:)rotection and free locomotion. 

 Modiolaria marmorata buries itself in the tough test of an 

 Ascidian. 



Amongst Gastropods are many examples of associated forms 

 which may probably be considered as instances of Commensalism, 

 although the nature o£ their food being undetermined, they may 

 perhaps prove to be parasites, no apparent injury to their hosts 

 being evident. 



Thus Capiilus crijstallinus is often found attached to the under 

 or actinal surface of the starfish LineJcia multiformis ; Styliger 

 Turtoni on ouv covimon. P^cTiinus miliaris ; Ampliiperas aciciilaris 

 on a Gorgonia, Pterogorgia pinnata. It is interesting to notice 

 that the shell of this Gastropod has a similar purple colour to 

 the Gorgonia. 



To turn now to fish. The various species of Fierasfer are well 

 known to seek shelter in the interior of sea-cucumbers : thus 

 F. acus may often be seen in the aquaria at Naples hovering 

 about Solothuria tuhulosa, and on the slightest alarm plunging 

 head first through the mouth or anus of the Echinoderm, to 

 emerge again when its enemy has departed. Stromateus micro- 

 chirus and the young of many other fish may constantly be found 

 swimming under the umbrella of the larger Medusae, the urti- 

 cating nature of their host defending them against their numerous 

 enemies. Mr. Saville Kent has also called attention to two 

 species of Amphiprion, viz. A. bicinctus and A. Kenti, which are 

 only found swimming fi-eely in the digestive cavity of a large sea- 

 anemone, the first in Discoma Eaddoni, the second in D. Kenti. 



We will now turn to some instances of Symbiosis, that is, of 

 forms associated for mutual advantage. In Lichens we have 

 familiar examples of such association, the nature of which was 

 first pointed out by Schwendener, his views being accepted by 

 most, although still denied by some lichenologists. According 

 to him the green cells, the gonidia of lichenologists, are algfe 

 which can live independently, but in the lichen are associated 

 with a fungus which can only maintain its life by their aid. 

 Again, the nodes on the roots of the Leguminosae are due to a 

 fungus by whose aid the higher plant is believed to be able to 

 avail itself of atmospheric nitrogen. 



