418 NOTES. 



tion with broken points, and this could not be the case if they were 

 formed in the horny fibres by the creature itself. Since the horny 

 tissue, even in living sponges, is much too tough passively to allow of 

 the intrusion of foreign bodies during the life of the sponge, their being 

 imbedded in the fibre can only result from the sponge drawing them to 

 itself by a voluntary act and including them between the layers at the 

 growing apex of the horny fibres. One of the most remarkable 

 instances of the utilisation of foreign bodies as an integral constituent 

 of an animal structure is offered by the species of Xenophora among 

 the mollusca ; they voluntarily incorporate into their shells, in regular 

 order, other shells and fragments of stone or of coral. Their intimate 

 adhesion shows that this combination must be effected before the shell 

 has hardened, and each particular species of Xenophora eeems also to 

 have more or less choice among the materials at its disposal. Bergh 

 has recently shown that Staurodwis Januarii, a naked marine mollusca, 

 eats the spiculas of sponges and deposits them in its skin. 



We may here allude to the use made by the larvae of the Phry- 

 ganeidcs of leaves, snail- shells, roots, &c., in building their cases 

 (caddis-worms). 



The above-mentioned regular association of two kinds of animals, 

 one of which lives in or on the other as a constant parasite, offers many 

 remarkable phenomena. It is known, for instance, that the hermit crab 

 ( Pa-gurus) is sometimes infested by dark-brown Crustacea of the group 

 of the RkboeepJu&t ; when these are present the female germ-glands 

 never develope in the host, and eggs and parasites are never found 

 together in the same individual. In certain localities the parasites are 

 so common that out of hundreds of hermit crabs scarcely a single 

 individual will be found without one, although their numbers are no 

 less than in other spots in the vicinity where there are no parasites, or 

 hardly any. This proves that the growth of the Pagurus is not in the 

 least hindered by the foreign parasite, while the development of sexual 

 maturity is wholly arrested ; and we see, moreover, that the conditions 

 for the reception of the parasite or for its avoidance may be quite dif- 

 ferent in two contiguous spots. 



I myself have made similar observations with regard to Lymneea 

 itagnalis. The larvae of Trematoda which infest these water-snails, in 

 the first place at any rate, and almost exclusively, as it would seem, 

 destroy the germ-glands, but they do not check the creature's growth. 

 In however great a number the parasites may be present, the mollusc 

 grows all the same, but propagation is completely prevented. How far 

 the sterility thus induced in certain individuals may possibly give rise 

 to other changes in their external or internal conditions is wholly un- 

 known, and has not been investigated. The larva of a fly, Cuterebra 

 cmasculator, destroys the testes of various American species of squirrel 

 without affecting the other vital functions ; and the number of such 



