382 PARASITES. 



of the Leech, we are warranted to bring in a verdict against 

 it of conspiring the death of the animal that harboureth it, 

 by sucking out its vital juices. 



The preceding are external parasites : a more numerous 

 host infest the viscera and intestines, but I intend to pass 

 them over slightly, for they either do not influence the 

 health and habits of the Mollusks they live and feed upon, 

 or that influence has been unmarked. — Dujardin mentions 

 two infusorial animalcules (Trichomonas limacis and Alber- 

 tia vermiculus), whose habitat is the intestines of the slugs.* 

 The lacustrine pulmonated Mollusca are more profusely ver- 

 minous. More than one species of Distoma has been found 

 in their viscera; and, to judge from analogy, we should 

 suppose that the infected organs must suffer softening and 

 disorganization. M. Baer has also found a Filaria in the 

 abdomen of Limneus stagnalis ; and in many Mollusca of 

 the same family he has met with a worm allied to the Naides 

 living in the respiratory cavity, or hanging like little tufts 

 of thread from the sides of the abdomen, whence he has 

 named it Chcetogaster. f Besides these a kind of animal- 

 cules named Cercariae find an appropriate nidus for their 

 evolutions in the body of the lacustrine snails ; and the curi- 

 ous transmutations of form they undergo in the interior of 

 the Mollusk, and in the circumfluent water, afford one of 

 the most striking illustrations of Steenstrup's theory of 

 alternating generations. % 



The parasites of the fresh-water bivalves have furnished 

 M. Baer with subjects for a copious essay.§ One of them 

 (Aspidogaster conchicola) lives in the pericardium of Unio 

 pictorum, and of several Anodontas ; another worm was 

 detected within the auricle of the heart swimming in the 

 blood ; and the species (Distoma duplicatum) to which this 

 individual belonged swarmed populous in every part and 

 viscus of the body of Anodonta ventricosa. Another para- 

 site named Bucephalus polymorphus, from its likeness to an 

 ox's head and its multiformity, breeds and multiplies in the 

 liver and interior of these devoted mussels, which thus may 



* Hist. Nat. des Infusoires, 300 and 654. 



f " La cavit6 respiratoire et le rein du Limneus stagnalis ont fourni un 

 autre entozaire nouveau, rentrant, par son organisation, dans la classe des 

 Annflides, et voisin des Naides. Les paquets de soies que ce ver porte 

 par paires, sur les cotes de la surface abdominale, lui ont fait donner le 

 nom de Chtetogaster. 11 s'est retrouve dans le Planorbis corneus et dans 

 beaucoup d'autres Mollusques d'eau douce. On les rencontre aussi a l'etat 

 libre dans les eaux habitees par ces Mollusques." 



1 See Agassiz and Gould's Prin. of Zoology, i. 130. 



§ Bull, des Sc. Nat. (Fev. 1829) xvi. 292-297. 



