234 Transactions of the ^r.^ll^^m,T^,'' 



1850, shortly describes it among twelve other species, under the 

 name of " Malleolus furcatus Ehrenherg. Corpus elongatum de- 

 pressum, Os subterminale, Acetabulum centrale tubuliforme, Cauda 

 teres simplex, Apice fureata. Habitaculum Limnaeus stagnaHs." 



Siebold described a species of cercariae, which he says are to be 

 seen in thousands upon specimens of large fresh- water snails, the 

 body of which is of an elongated form, head triangular, and ventral 

 sucker scarcely visible. The swimming movements are restless and 

 characteristic, and chiefly performed by the tail. The tail tapers 

 from the body downwards, but is not acutely terminated. This 

 member is thrown off in its change to the fluke. It is, I beheve, 

 the remarkable way in which these creatures when in a confined 

 space are seen to break off this appendage, which has led to a behef 

 that the tail is not absorbed into the body of the future animal in 

 its subsequent metamorphosis, as we know occui's in the analogous 

 transformation of the tadpole into the frog. I venture to think 

 Siebold's description in this particular an error. Some naturahsts say 

 the ova of cercarise are developed within the body of the snail ; this is 

 scarcely consistent with what is stated regarding the healthy state of 

 infested snails. I beheve what has been seen are simply gregarinae, 

 which are sometimes found in abundance in the ahmentary canal of 

 the Lymnaeus. The parent may find it convenient to conceal her eggs 

 in the folds of the mantle, or in the pulmonary cavities, where they 

 will be secured fi'om the attacks of enemies; or they may be at- 

 tached to the internal portion of the shell, just as vorticella are to 

 the outer. I am of opinion the eggs are hatched here that the young 

 animals may find a ready means of subsistence ; a careful nurse, 

 who gently carries them among plants wliich afford an abundant 

 supply of food, and at a certain period of growth is sure to deposit 

 them where they instinctively await the thirsty mammal, whose 

 stomach they must occupy before they attain to a more perfect stage 

 of development. 



The nervous trunk runs continuously through the tail and body, 

 and when the animal can be kept sufiiciently still is seen to present an 

 unbroken chain. The remarkable bi-furcated tail-like process enables 

 this species of cercaria to move about with extraordinary rapidity 

 of action ; it is so articulated to the body, that it can be brought 

 up to a very acute angle, and when broken off the broken extremity 

 presents a concavity in which the convexity of tail accurately fits. 

 The body and tail are equally active when detached from each other, 

 and continue to swim about as if nothing unusual had happened. 

 When exhaustion commences, the contents of the stomach are 

 ejected through the broken extremity or the mouth, and therefore I 

 was unable to satisfy myself whether the nipple-like process placed 

 near the centre of the body is used as a vent. This nii^ple is pecu- 

 liar to the species. The larva always attaches itself by this 



