in STROBILATION AND SEGMENTATION 175 



as that most advantageous for the preservation of the race, and would finally become 

 the normal mode of infection. 



In the systematic review the intermediate hosts of several Trematoda and Cestoda 

 are given, as well as the final hosts. The biological relations between the host and the 

 intermediate host can easily be recognised, especially in the case of the Cestoda. 



Occasionally there are two intermediate hosts in the normal course of life. Free- 

 living young forms, e.g. the ciliated larvse of the Trematoda, often effect the transi- 

 tion of the parasite from host to intermediate host, or, as the Cercarice, from inter- 

 mediate host to definitive host. 



In the Cestoda it is possible for parasitism to thrive to such an extent by the 

 passive transmission of various stages of the parasite from host to host that the 

 animals never lead a free life. The degenerating influence of the parasitic mode of 

 life has here told upon all stages of development. 



However refined the artifices for infecting new hosts may be, the result of the 

 process must always^ to an extraordinary extent depend upon chance. It is a chance 

 when the egg or the embryo of the Distoma hepaticum reaches the water, a chance 

 when it meets a Limna&us truncatulus, a chance when the encysted Cercaria, with the 

 plant on which it lies, is eaten by a sheep. Thousands and thousands of eggs thus 

 miss their aim. There is therefore another way of providing for the maintenance of 

 the race in parasites, viz. their extraordinary fruitfulness and their highly 

 developed capacity of reproduction. This capacity is very easy for them, because 

 the conditions of existence in which they find themselves are the most favourable 

 possible. A Distoma, indeed a single proglottis of a Tcenia, is capable of producing 

 thousands, or even hundreds of thousands, of eggs and embryos. And if of all these 

 eggs but 1 or 2 on the average reach their aim, the maintenance of the race is 

 provided for. Propagation by gemmation comes to the assistance of sexual propaga- 

 tion by fertilised eggs in the segmented tapeworms and in the young forms known 

 under the names of Echinococcus and Coznurus, 



In those cases also, in the Trematoda, where generations living in the so-called 

 intermediate host are not surrounded by conditions so favourable that they can 

 develop into adult Trematoda with male and female organs, they still possess the 

 capacity, in spite of their reduced condition, of producing at an early stage a sort of 

 egg, the germ cells; these dispense with fertilisation and nevertheless develop 

 (parthenogenetic reproduction of the Sporocysts and Redice). 



When we consider the degenerated condition of the Sporocysts there is some 

 justification for assuming that the Dicyemidce and Orthonectidce (cf. p. 60), which are 

 very similar to these Sporocysts, are degenerated Trematoda from whose life-history 

 the typical Trematode generation has completely disappeared. 



Strobilation and Segmentation. 



We have seen that the bodies of most Cestoda are segmented, and 

 we have shown this segmentation to be the result of an axial budding 

 or strobilation. The whole segmented body is thus an animal stock. 

 In a few Turbellaria, especially in Gunda, segmentation also occurs, but 

 in quite another way ; this is the regular paired arrangement of the 

 organs which in the Polydada and Triclada are generally present in 

 considerable numbers. There is a repetition at regular intervals of 

 the transverse commissures of the nervous system (the ladder nervous 

 system), the male and female sexual glands, the lateral intestinal 

 branches, the dissepiments lying between them, and the external 



