130 COMPARATIVE ANATOMY. 



stage of development, and form more or less independent individuals. 

 This process represents therefore a process of gemmation, the pro- 

 duct of which is the Tsenia-chain ; each separate joint is a metamere 

 as compared with the general organism of the chain, but it is also 

 to be regarded as a separate person, since it is capable of an indepen- 

 dent existence, the slight duration of which is explained by its struc- 

 ture, which is adapted to a parasitic habit. The metamerism of the 

 body, seen in the Cestoda, may be derived from a process of gem- 

 mation, and gemmation itself is correlated with the elongation of 

 the body. It is an intermediate stage between the two phgenomena ; 

 and there is, therefore, no well-defined antagonism between them. 

 Where metamerism is faintly expressed, it becomes more and more 

 nearly a case of simple elongation. 



In many divisions we may find examples of this incomplete 

 metamerism. It is indicated in various systems of organs in the 

 Nemertina. In the Gephyrea also, metamerism is not a general 

 phEenomenon, for several systems of organs are not affected by it. 

 On the other hand, it obtains generally in the Annulata, and gives to 

 the organism a multifid appearance. In them it is not unfrequently 

 associated with a process of distinct gemmation. In the embryonic 

 body there are generally fewer metameres than in the adult. The 

 freshly-developed segments are formed in front of the last one. The 

 elaboration of particular metameres gives rise to a large number 

 of modifications. Such also result when a number of metameres 

 undergo concrescence, and the primitive arrangement is only indicated 

 by certain systems of organs ; this gives rise to conditions which 

 it is difiicult to distinguish from those in which metamerism is 

 just commencing. When metameres are developed the organism 

 becomes one of a higher grade of organisation, although indeed 

 this is not the only path towards such a stage, for we meet with 

 differentiations of other kinds which lead to higher conditions. The 

 more definite differentiation of the ventral surface owing to the 

 development of a groove, as in the Solenogastres (in Chastoderma 

 this is found in the posterior region of the body only) is an example 

 of this ; it represents the first stage in the formation of that pedal 

 surface of the body which is seen in the lowest MoUusca. 



§ 102. 



There are various other modifications in certain smaller divisions, 

 which are to be attributed to adaptations to changed external 

 conditions of life ; this is especially the case in the entoparasitic 

 Platyhelminthes. The " cystic form " must be regarded as the 

 most important of these modifications ; this, which is intercalated 

 into the developmental history of the Cestoda, is, in its phylogenetic 

 liistory, just as certainly due to the organism having entered into 

 relations which at first were strange and abnormal to it, as is the 

 general parasitism itself referable to habits, which were only 

 secondarily acquired. The phylogenetic history of the cystic form 



