62 WORMS PARASITIC IN MOLLUSCA chap. 



well as with Succinea putris^ Limax agrestis and maximus^ Avion 

 ater and hortensis. 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 touch- 

 ing 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 pref- 

 erence for the branchial chamber ; those which enter the foot or 

 other outlying parts of the Limnaea 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 (^Rhahditis) are known to inhabit 

 the intestine of Arion^ and the salivary glands of Limax agrestis. 

 Diptera habitually lay their eggs within the eggs of Helix and 

 Limax. Many species of mite (^Acarina^ infest land Pulmonata. 

 No adult Limax maximus is without at least one specimen of 

 Philodromiis (?) limacum^ and the same, or an allied species, 

 appears to occur on the larger of our Helices^ rt tiring upon 

 occasion into the pulmonary chamber. 



Several of the Crustacea live associated with certain molluscs. 

 Piyiiiotheres lives within the shell of Pimia^ Ostrea^ Astarte, 

 Pectunculus^ and others. Apparently the females alone reside 

 within the shell of their host, while the males seize favourable 

 opportunities to visit them there. A specimen of the great 

 pearl-oyster QMeleagrina margaritiferd) 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.^ Ostracotlieres Tridacnae lives 

 in the branchiae of the great Tridacna. A little brachyurous 

 crustacean inhabits the raft of lanthina, and assumes the brill- 

 iant 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. 



2 II. Woodward, P. Z. S. 1886, p. 176. 



