PLATHELMINTHES 



199 



out from the Cysticercus by non-sexual methods, and then itself sepa- 

 rates by division into proglottides, each sexual generation, the product 

 of which is the embryo (Cysticercus), is followed by two non-sexual 

 generations. On the other hand, in view of the circumstance that in 

 all probability the continuity of the individual persists, the course of 

 development in the tapeworm has more recently been explained as a 

 metamorphosis (Gkobben, No. 4 ; Claus, No. 3). Certain very simply 

 organized tapeworms, such as Archigetes, and a tapeworm living in the 

 body-cavity of Cyclops (Leuckart, No. 7 ; A. Grubek, No. 5), are evidence 

 in favour of this view. These Cestodes appear to reach the permanent 

 condition without first passing through the typical Cysticercus stage. 

 The one last mentioned is converted directly into the sexually mature 

 animal ; the other is metamorphosed into the sexual animal, simply by 

 its body becoming separated into an anterior and posterior portion, 

 whereby the worm acquires a Cercaria-like appearance (Leuckart, No. 

 7). If one considers the posterior portion of the body as equivalent to 

 the bladder of the Cysticercus, this tapeworm arrives at sexual maturity 

 even in the Cysticercus stage. 



Like Archigetes, the unsegmented CaryophylUnis, which is provided 

 with a single set of sexual apparatus, represents throughout life a stage 

 which is equivalent to the scolex of other tapeworms, together with a 

 single accompanying segment. Therefore the development of the 

 embryo into the scolex would correspond to a metamorphosis, in which, 

 however, it is to be noted that with the bladder are cast off parts of the 

 body which originally represented the body of the entire individual. But 

 a similar state of affairs exists in the origin of the Nemertean from the 

 Pilidium, and the starfish from the Bipinnaria, without our calling these 

 processes alternation of generations. 



As regards the second process of non-sexual reproduction — namely, the 

 division into proglottides — those cases are particularly noteworthy in 

 which, as in some Acanthobothridce and Echenelbothridce, the proglottides 

 after detachment are able to live for a long time, and increase to 

 several times their former volume. They give the impression that one 

 has to do with independently living individuals resembling somewhat 

 Distomuin. However, one must consider even here the earliest origin 

 of the Cestodes, and go back to forms which, like CaryophyUaus and 

 Amphiptyches, exhibit only one set of genital ajjparatus. They might be 

 traced back through transitional forms like Amphilina (comp, infra, 

 p. 201) to forms resembling Trematodcs. In the beginning only one set 

 of sexual apparatus was present, later numerous sets made their appear- 

 ance, and this condition led, by reason of its advantage, to the detach- 

 ment of individual segments of the body. The Ligididir may perhaps 

 give us some foothold in this connection. Even if the conditions which 

 we find in them are to be considered as regressive, they may, neverthe- 

 less, be looked upon as reversions to an earlier condition. In the 

 Ligidida the genital organs are repeated without the appearance of an 



