CASES OF CONSTANT ASSOCIATION. 333 



to "whom he offers a residence though he cannot be said to 

 invite them. 



The modifying influence of living organisms on living 

 animals. In a former chapter we have already become 

 acquainted with a case coming into this category ; namely, the 

 cysts or galls formed on certain corals by the presence of 

 crabs. In these cases the crabs do not seem to be particularly 

 affected by their host, unless we are disposed to attribute the 

 flat shape of Hapalocartinus or the cylindrical form of Litho- 

 scaptus to the direct mechanical influence of their peculiar 

 dwelling-places. The corals, on the contrary, exhibit such great 

 and peculiar deviations from their normal growth that the 

 effects of the parasites on their host are plainly perceptible, 

 both as stimulating and as checking its growth. There are a 

 not inconsiderable number of such cases known. Certain sea- 

 spiders, Pycnojonidce, produce exactly similar galls, but com- 

 pletely closed (see fig. 85), on the stems of a small polyp as 

 Hodge informs us. All the larvae of our fresh-water mussels, 

 after leaving the parent, require to attach themselves to the 

 skin of a fish before they can develope any further ; there they 

 occasion an excrescence which gradually swells to a capsule 

 visible even to the naked eve, and in this cavity the larva lives 

 for months and goes through its metamorphosis into a true 

 bivalve. "We must include in the same category the gall-flies 

 forming galls on extremely various plants. 



"We are now accustomed, in all such cases of deviation from 

 the normal growth, to regard them pathologically, as the result 

 of disease, and certainly not altogether erroneously, since we 

 know that they constitute more or less frequent exceptions. 

 But supposing that the reciprocal relations between two animals 

 or an animal and a plant were of such a character that each 

 was dependent on the other in an equal degree, so that neither 

 could exist without the other, any deviation from the normal 

 growth must evidently no longer be regarded as indicating dis- 

 ease. "We must even consider the apparent abnormity as a 

 peculiarity or character of the species, since it must necessarily 

 occur in every individual of the species. The constancy of the 

 causes which first led to the association of the two kinds of 



