PARASITES OP ANIMALS. 55 



and is supposed to perform the office of excretory organs, 

 analogous to the kidneys and liver of vertebrate animals. In 

 the central region of the body there is a well developed repro- 

 ductive system, both male and female organs being generally 

 contained in the same individual worm ; but in some species 

 the sexes are separate. All the rest of the body, around these 

 organs, is composed of a rather firm and solid tissue, the or- 

 gans not being contained in a distinct cavity. 



The species constituting this order are very numerous, be- 

 longing to several distinct families and many genera. About 

 400 species have been described. They are found in all classes 

 of vertebrate animals, and are numerous in their larval states 

 in many invertebrates, especially in fresh-water snails. They 

 particularly abound in fishes, frogs, and aquatic birds. Nearly 

 all the species undergo very remarkable transformations, with 

 alternate generations, some of them passing different stages of 

 their lives free in water, and then in two or more distinct an- 

 imals, as parasites. So that the adult forms, found in the 

 higher animals, are generally derived from larvae swallowed 

 in small moUusks, like the river snails, and perhaps in insects. 



Ill, AcANTHOCEPHALA. (Thorn-headcd worms.) 



This order includes elongated, more or less cylindrical 

 worms, usually with the body encircled by distinct transverse 

 ridges and wrinkles, and which have at the anterior end a 

 prominent, elongated or conical, retractile proboscis, covered 

 with numerous recurved hooks, by means of which they at- 

 tach themselves to the mucous membrane in the intestine of the 

 animals that they inhabit. The body contains a large cavity, 

 in which the reproductive organs are contained. The sexes are 

 distinct. The ovary, situated in the anterior part of the body, 

 sets free large roundish masses of cells from which the ova are 

 afterward developed, while these masses, or " cocoons," are 

 free in the abdominal cavity, each mass containing a large 

 number of eggs, often several hundred. In the male the two 

 testicles are attached to the lower closed end of the digestive 

 sac, and are connected by ducts with the intromittent organ, 

 which is a slender spiculum, enclosed in a sheath, situated at 

 the posterior end of the body. 



