62 WORMS PARASITIC IN MOLLUSCA cuap. 



Not one of them would it touch, except occasionally very young 

 specimens of L. peregra, and in these its development was arrested 

 at an early stage. But on touching a L. truncatula the embryo 

 seems to know at once that it has got what it wants, and sets 

 to work immediately to bore its way into the tissues of its 

 involuntary host, making by preference for the branchial chamber; 

 those which enter the foot or other outlying parts of the Zimnaea 

 proceed no farther.^ 



Many similar cases occur, in which littoral Mollusca, such as 

 Littorina and Buccinum, form the intermediate host to a worm 

 which eventually arrives within some sea-bird. 



Certain Nematode worms {Rhabditis) are known to inhabit 

 the intestine of Arion, and the salivary glands of Limax agi^estis. 

 Diptera habitually lay their eggs within the eggs of Helix and 

 Limax. Many species of mite (Acarina) infest land Pulinonata. 

 No adult Limax maximus is without at least one specimen of 

 Philodromus (?) limacum, and the same, or an allied species, appears 

 to occur on the larger of our Helices, retiring upon occasion into 

 the pulmonary chamber. 



Several of the Crustacea live associated with certain molluscs. 

 Pinnotheres lives within the shell of Pinna, Ostrea, Astartc, 

 Pectunculus, and others. Apparently the females alone reside 

 within the shell of their host, while the males seize favom-able 

 opportunities to visit them there. A specimen of the great 

 pearl-oyster {Meleagrina margaritifera) was recently observed 

 which contained a male Pinnotheres encysted in nacre. It was 

 suggested that he had intruded at an unfortunate time, when no 

 female of his kind happened to be in, and that, having penetrated 

 too far beneath the mantle in the ardour of his search, was made 

 prisoner before he could escape." Ostracotheres Tridacnae lives 

 in the branchiae of the great Tridacna. A little brachyurous 

 crustacean inhabits the raft of lanthina, and assumes the brilliant 

 blue colour of the mollusc. 



Means of Defence 



As a rule, among the Mollusca, the shell forms a passive 

 mode of resistance to the attacks of enemies. Bivalves are 



1 A. P. Thomas, Q. J. Micr. Sc, N. S., xxiii. (1883) p. 99. 

 - H. AVoodvvard, P. Z. S. 1886, p. 176. 



