182 THE ANATOMY OF INVERTEBRATED ANIMALS. 



enormously elongated. They are converted into ramified 

 tubes called sporocysts, which sometimes occupy all the inter- 

 spaces of the viscera of the muscle. These develop new 

 JBuce})hali by internal gemmation. The Trematode condition 

 appears to be the genus Gasterostomum, which inhabits fresh- 

 water fishes. 



The Sporocysts, Rediae, and Cercariae, free or encysted, 

 are found almost exclusively in invertebrated animals, while 

 the corresponding adult Trematodes are met with in the verte- 

 brated animals which prey upon these Jnvertebrata. 



The singular double-bodied Diplozoon paradoxwn has 

 been shown by Von Siebold to result from a sort of conjuga- 

 tion between two individuals of a Trematode, which, in the 

 separate state, has been named Diporpa. The Diporpce, 

 when they leave the egg, are ciliated and provided with two 

 eye-spots, with a small ventral sucker and a dorsal papilla. 

 After a time the Diporpm approach, each applies its ventral 

 sucker to the dorsal papilla of the other, and the coadapted 

 parts of their bodies coalesce. They acquire fully developed 

 sexual organs only this after union. 1 



Gyrodactylus multiplies agamically by the development 

 of a young Trematode within the body, as a sort of internal 

 bud. A second generation appears within the first, and even 

 a third within the second, before the young Gyrodactylus is 

 born. 



The Cestoidea. — The Tape-worms are all endoparasites, 

 and, in their adult condition, infest the intestines of verte- 

 brated animals. 



The simplest form known is Caryophyllceus,* found in 

 fishes of the Carp tribe. It has a slightly elongated body, 

 dilated and lobed at one end, so as to resemble a clove, 

 whence the name of the genus. In structure it resembles a 

 Trematode, devoid of any trace of an alimentary canal, but 

 provided with the characteristic water-vascular system and 

 with a single set of hermaphrodite reproductive organs. 



In Ligula, the body is much elongated, and, at the head- 

 end, exhibits two lateral depressions. It is not divided into 

 segments, but there are numerous sets of sexual organs ar- 



1 Zeller, " Untersuehungen uber die Entwickelung des Diplozoon paradox- 

 urn." (Zeitschrift fur wiss. Zoologit, 1872.) 



2 See the " Memoire surles Vers Intestinaux," 1858, by M. P. J. Van.Beneden, 

 to which I am much indebted for information respecting this and other genera 

 of Cestoidea which have not fallen under my own observation. Also Leuckart, 

 '* Die menschlichen Parasiten," 1863 ; and'Cobbold, " Entozoa." 



