194 



LOWER INVERTEBRATES. 



the middle of the sixteenth century, it was not until 1882 that the complete life history 

 was known, as we related above. They are very injurious to their hosts on account o'i 

 the interference with the discharge of bile, which easily becomes serious and even fatal 

 to their unfortunate entertainer. 



The genus Distomum is a very extensive one, comprising a great many species, in- 

 festing an almost incredible variety of animals ; indeed, it may be questioned if there 

 is any other genus of living things privileged to be such a universal infliction. Man 

 alone is exposed to the attacks of no less than five different species, while the poor frog 

 is even more numerously endangei-ed. Molluscs suffer especially from the attacks of 

 the Distomum in its larval state, so that it is not at all uncommon to find the whole body, 

 of a pond-snail for example, crowded with sporocysts or redia?. The following are the 

 species which attack man: Distomum crassum, occurring in China — -it inhabits the 

 intestine and grows to one or two inches in length; D. lanceolatum, which occurs to- 

 gether with the true river fluke, but is much shorter, and, proportionally, much narrower, 

 so that it can be very readily distinguished from the hepaticum ; next a doubtful 

 species which has been only once observed, D. opthalmobium ; and, finally, the D. 

 heterophyes, known only in Egypt. The genus may be readily recognized by the two 

 ventral suckers, which lie near together at the anterior end of the body. 



The Trematoda are divided into two 

 orders, the Distomeae and Polystomeae. 

 The former comprises those forms that are 

 related to the genus Distomum, and have 

 two or sometimes only one sucker, while the 

 latter have two lateral small suckers at the 

 anterior end of the body and one or several 

 suckers posteriorly, while connected with 

 the latter are often several booklets. In 

 the first order there reigns a considerable 

 similarity of appearance, while in the sec- 

 ond there is an unusual degree of diversity. 

 Among the Polystomeae, however, is one spe- 

 cies which offers the wondering naturalist 

 an unparalleled phenomena — two separate 

 and comjslete individuals united in one. 



The Diplozoon paradoxum, to which 

 the closing sentence of the previous para- 

 graph referred, is indeed well named, for 

 it is literally two animals united in one. 

 There are two bodies precisely resembling 

 each other in every particular, and united, 

 like the Siamese twins, by a narrow communicating band, so as to form but one ani- 

 mal, the nutrient canals of one division communicating freely with those of the oppo- 

 site half. One might easily regard this extraordinary arrangement as an accidental mon- 

 strosity, but observation has proved it to be common to all the individuals of the 

 species. The animals, which are of very small size, being not more than two or three 

 lines in length, are found attached to the gills of the bream, from which they derive 

 nutriment. Siebold discovered that the Diplozoon arises by the union of two distinct 

 worms, and the whole life history was subsequently worked out with great exactitude 



Fig. 175. — Diplozoon paradoxum ,• a, mouth; 6, ante- 

 rior suckers; c, stomach: /, oviducts; .(?, uterus; i, 

 testis; k, vas deferens; 4 vascular caiials; m, pos- 

 terior suckers. 



