142 HABITS AND ECONOMY OP THE MOLLUSC A. 



small bivalves entire (Donax, Mactra, Cardium), and dissolves 

 the animal out of its shell ; the digestive cavity of the Actinias 

 nearly always contains gastropods (Bulla, Nassa, Trochus, 

 Rissoa, etc). 



A number of other animals live parasitic upon mollusks. 

 There are several crustacean parasites of Doris ; the lamelli- 

 branchiates are frequently infested with the Pinnotheres ; a little 

 crab of this genus is frequently found enclosed in the valves of 

 the American oyster ; the marine mussel (Mytilus) often 

 nourishes a similar guest ; Ostracotheres Tridacnae takes hold 

 of the branchiae of Tridacna (Riippell), and«a little brachyurous 

 crustacean, of a brilliant blue color, is nearly always found 

 attached to the raft of Ian{;hina. 



Pulmoniferous gastropods are infested by little acarides. The 

 Philodromus limacuvi lives in the pulmonary chamber of Euro- 

 pean limaces and snails; and a similar species, Hypopus concolor ^ 

 Hald., is found abundantly upon American Helices. But the 

 Vermes furnish the largest contingent of parasites on Mollusks : 

 unfortunately for science, we scarcely know anything concerning 

 the cycle of their migrations. The following example will 

 serve to show the interest attending such researches : The 

 sporocysts or germinative sacs of Distoma retasum produce, 

 in the Limna^ans, the Cercarise, or tailed larva, without genital 

 organs. These Cercarise are encysted in the interior of neurop- 

 terous larva. Finally they complete the cycle of their trans- 

 formations in the frog, where under the form Distoma they 

 acquire sexual organs. Several other forms, larval in aquatic 

 mollusca, and perfect in frogs and aquatic birds, are known to 

 science. 



Aspidogaster conchicola lives in colonies in the pericardium ' 

 of Unio and Anodonta, and Cotylaspis insignia occurs in the 

 upper branchial cavit}^ of the latter genus (Leidy). The Palu- 

 dina vivip)ara of Europe nourishes no less than eight species of 

 minute parasites. Prof. Haldeman, and others, have described 

 several parasites upon American fresh-water mollusks ; among 

 them is Anoploplirya ve7'micularis, which Dr. Leidy observed in 

 the intestine of P.aludina decisa^ sometimes so abundant as to 

 distend the intestine. 



Among terrestrial mollusks, Succinea lodges, in the interior 

 of its tentacles, a sporocyst called Leucochloridium jjaradoxum^ 

 which is transformed in various birds into Distoma macrostomum. 

 The tentacle thus occupied is swelled far beyond its usual size. 

 Bauclon states that the wagtail opens these tentacles for the 

 purpose of eating the larva. Dujardin has seen several species 

 of Distoma in the viscera of Helix and Limax. 



The Trematodse of marine mollusks are less known ; they 

 have been found in Buccinuni undatum and Littorina litorea. 



