46 ZOOLOGY. 



into rings, and bearing sonio resemblance to a leech, but sometimes so short 

 as to be circular. 



1. The Trematoda are internal monoicous parasites, having an anterior, 

 suctorial opening, and one or more suctorial disks of attachment, which 

 afford generic characters. Dlstoma (or Fasciola) hepatimm {pi. "11^ fig. 36), 

 which is a good example, is about an inch long, infests the gall-bladder, 

 liver, and rarely the neighboring veins in man, sheep, oxen, deer, gazelles, 

 camels, goats, horses, and hares. In sheep it is the cause of the fatal 

 disease named rot. The severe winter of 1841-2 in Germany, was fol- 

 lowed by the death of many deer, which were found to be much infested 

 with Distoma. 



Fresh-water snails of the genera Planorbis and Limnea are infested by a 

 minute animal, with a globular body and slender tail, resembling a tadpole, 

 and forming the supposed genus Cercaria, of which two American species 

 were published in 1840, the motions of which are similar to those of their 

 European analogues, the tail being rapidly thrown into the shape of an S, 

 and easily detached.* Steenstrup has in some measure cleared up the his- 

 tory of the Cercaria, which are the larvae of Trematoda. After swimming 

 about freely for some time, they attach themselves to the outside of the 

 snail, and settle in the mucus of the exterior, maintaining themselves by an 

 abdominal sucker, and in the course of their movements losing tlie tail, a 

 loss which gives them somewhat the appearance of a Distoma. They now 

 enter the pupa state, in which they remain for some months without appar- 

 ent change. They afterwards acquire spines anteriorly, and such individu- 

 als were found within the snail. C. Th. Siebold thinks the Distoma is 

 iinally developed in the water-fowls which swallow the snails, an analogous 

 fact having l)een observed by Creplin, who found a species in a stickleback 

 fish, and also in water-foNvls. 



2. The Planariidm contain a number of small leech-like animals, found 

 both in fresh and saltwater, which glide along like a snail over solid objects, 

 or, passing up an object to the surface, they creep along this with the back 

 downwards, and the belly attached to a thin film of water. The single 

 opening to the ramifications of the stomach is usually about the centre of 

 the inferior surface; and whatever is taken through this that is indigestible, 

 is snbsequently rejected by sucking in a quantity of water, and ejecting the 

 whole together. 



Planaria {Planocera) cormita {2)1. 77, fig. 35) has two horn-like 

 extensions anteriorly. P. {Pendroccelum) gracilis, Hald. 1810, is three 

 fourths of an inch long, and one tenth broad, fuliginous, veined with black ; 



* C hyalocaiida, Hald. Body dark brown, or blackish, about as long as the tail ; tail trans- 

 parent, tapering, and suddenly diminislied at its junction witli the body. Just 'visible to the 

 naked eye. Parasitic upon Physa heterostropha. Susquelianna, 



C. bilineata. Perfectly white. Microscopic characters : Head and tail translucent ; body Avith 

 two dark longitudinal lines, wliich have a tendency to connect, so as to form a circle when the 

 animal contracts ; there is a light posterior circular spot occasionally visible, the tail is shorter 

 than the extended body, and is not contracted at the base. Exceedingly numerous upon speci- 

 mens of Limiica catascopium, collected at Camden on the Delaware. 

 250 



