334 THE NATUEAL HISTORT EEVIEW. 



has added a sixth order, Oordiacea, which some unite to the Nema- 

 toidea. The order Cystica has recently been suppressed, since it has 

 been proved that an intimate genetic relationship subsists between 

 these forms and the Cestoidea. 



There exists, indeed, a greater amount of morphological resem- 

 blance among the Helminths than might, a priori, have been pre- 

 dicted. AYe must not, however, forget the existence of internal 

 parasites belonging to other classes, as the Infusoria and Insecta. 

 We find, on the contrary, some non-parasitic forms among the 

 Scolecida, namely, all the Tiirbellaria, and several genera of 

 Nematoidea* 



The astomatous Helminths {Cestoidea and AcantJiocephala) are 

 parasitic during the entire period of their existence. The parasitic 

 Nematoids, which form the majority of that order, spend a portion of 

 their life free. So do, likewise, the Gordiacea, and, to a still greater 

 degree, the Trematoda. 



The Nematoidea are directly developed from fertilized ova, and, 

 so far as is known, do not exhibit the phenomena of agamogenesis, 

 or asexual generation. The same is true of the Gordiacea, some 

 of which, however, undergo a slight metamorphosis. Some, also, 

 of the Tremaioda develop themselves, in the usual manner, from 

 ova of relatively large size, containing a copious supply of formative 

 material, "f 



But many Trematoda, and all Cestoidea and Acantliocepliala% 

 present in their developmental cycle singular changes, in which 

 metamorphosis is strangely blended with the phenomena of ' alternate 

 generation.' The primitive zooid resulting from the immediate 

 development of the embryo is, in all cases, sexless, and gives rise by- 

 gemmation to the organism in which true generative elements make 

 their appearance. Moreover, in order that development may take 

 place, it is absolutely necessary that the ova or zooids of each parasite 



\i\^Q^t Scolecida {Nematoidea) are those which approach nearest the Anmilata. 

 The Rotifera, a dioecious group, resemble the larval forms of the dieecious AnnelirJa. 

 In like manner the Turbellaria aprocta come next the Trematoda ; and the Ces- 

 toidea immediately precede the Acantlwcefhala, although placed in distinct sub- 

 classes. The Nematoids and Turbellaria proctucha complete the circle of Scolecida, 

 both groups touching upon the A mielida. 



* For the most recent information on these, sec Eberth, ' Untersuchungen iiber 

 Nematoden,' 1863, and Mr. Bastian's forthcoming memoir in Linn. Trans. 



t See Leuckart, op. cit. sup., p. 489. 



t Presuming the facts observed by Leuckart in the case of Echinorhyyichus 

 proteus to be true of the whole group. 



