ARTICULATA. 271 



natural history, which still remain to be cleared up, and from the 

 wonderful nature of those portions of the history of their development 

 which have been revealed by the recent researches of some of our most 

 eminent naturalists. 



The two parasitic orders are the Cestoidea, or Tape-worms, with 

 long jointed bodies ; and the Frematoda, with short elliptical or discoid 

 forms. 



Two species of Cestoid worms inhabit the human intestines, the 

 Tcenia solium and the Bothrioceplmlus latus. The former is the ordi- 

 nary Tape-worm,ihQ second occurs only in particular countries, in 

 Holland, Poland, and Switzerland, In the Cystic form some of these 

 worms do great injury to domestic animals ; one of the most noxious 

 is the so-called Ccenurus cerebralis, inhabiting the brains of sheep. 

 Many interesting facts are connected with the history of these animals ; 

 but our space forbids us from entering into further details. 



In the Distomidce, the animals possess two suckers, of which the 

 anterior contains the mouth. Of these the Fluke (Distoma hepaticum), 

 which infest the livers of sheep, is a well-known example. Other 

 species live in the intestines, and even the eyes, of other animals. 



The Tristomidce are furnished with three suckers ; two small ones 

 at the anterior extremity, between which the mouth is situated, and a 

 larger one at the hinder extremity. These worms principally infest 

 the gills of fishes, as do also Polystomidce, characterised by the presence 

 of several suckers at the hinder extremity of the body, whilst the 

 anterior extremity is either entirely destitute of those organs, or only 

 possesses a small one, in which the mouth is situated. This family 

 includes the singular Diplozoon paradoxuin, an animal which appears 

 to be compounded, like the Siamese twins, of two perfect individuals, 

 each containing precisely the same organs. 



The Gordicea, or Hair-worms, are at once distinguishable by the 

 extraordinary length of their bodies, which frequently present a close 

 resemblance to a horse-hair ; so close, indeed, that in former times the 

 popular belief ascribed their origin to the introduction of horse-hairs 

 into the water in which they are found. One of the most singular 

 circumstances connected with their history is, that if by any chance, on 

 breaking out of their insect-home, they find that dry weather has 

 produced a state of things incompatible with their notions of comfort, 

 they quietly allow themselves to be dried up, when they become per- 

 fectly hard and brittle ; but, strange to say, the moment a shower of 

 rain comes to refresh the earth with its moisture, the dormant Gordii 

 immediately recover their activity, and start off in search of their prey. 



