352 Prof. P. J. Van Beneden on the Litestinal TForms. 



cies of fish ; when once this is effected, they lose their tail and 

 cover themselves with a cyst. The fish which affords them an 

 abode, when swallowed by some carnivorous animal, is dissolved 

 in its stomach ; and the Cercaria, which was concealed in its 

 cyst, awakes as a Distoma in the stomach or intestine of its new 

 host. It is only in this last abode that the Distoma attains its 

 fully-developed sexual organs. Thus the Cercaria undergoes a 

 metamorphosis, like an insect ; but the Cercaria itself was pro- 

 duced from a Distoma, not by metamorphosis or change of form, 

 but by Generationswechsel or change of generation. 



The fifth part of the work is dedicated to the consideration of 

 the transmissions or migrations of intestinal worms. Our plan 

 does not permit us to dwell long on it. The interesting facts 

 relating to Tcenice are now generally known through the obser- 

 vations of Kiichenmeister, V. Siebold, Leuckart, and others. 

 It cannot be denied that our author, by his announcement that 

 the Tetrarhynchi of osseous fishes pass into Rhynchobothrius in 

 the stomach of cartilaginous fishes [Plar/iostomes), i. e. the Rays 

 and Sharks, has a claim to the discovery of the regular trans- 

 planting of worms ; it was shortly afterwards succeeded by the 

 remarkable discoveries respecting the Tceniie. 



The sixth and last part of the work before us contains con- 

 siderations respecting the systematic arrangement of worms. It 

 is well known that Van Beneden is not disposed to unite the 

 ringed worms {Annidata) with the Condylopoda, as Cuvier did, 

 by including in his type of articulate animals both these divi- 

 sions of the animal kingdom. He comes to the conclusion that 

 the animals which Linnreus named worms ought again to be 

 conjoined. This division of the animal kingdom would then 

 contain the Mollusks and Radiates of Cuvier besides the Annu- 

 lata. These animals, named by Van Beneden allocotyles, form, 

 according to him, six classes — Mollusks, Worms, Echinoderms, 

 Polyps (with which he unites the Acalephs), Foraminifera, and 

 Infusoria. The intestinal worms, no longer regarded as a di- 

 stinct class, are by him united with the so-named ringed worms 

 and TurbellaricB. He divides this class of worms into four groups 

 — Annctides, Nematdides, Phyllides, and Teretularides. To the 

 Nematdides belong, amongst the previously so-named Entozoa, 

 the Nematdidea and Acanthocephala ; to the Phyllides the Tre- 

 matoda and Cestdidea, which here are placed in the neighbour- 

 hood of the Leeches. 



The beautiful figures which are annexed to this work have all 

 been delineated by the author himself; they form not only an 

 ornament, but an essential part of it. In the last plate an ideal 



