iv VERMES STOCK FORMATION 267 



their organisation. We thus have before us an alternation of generations. A 

 mother animal which remains asexual produces asexually successive daughter 

 animals which differ from the mother animal externally ; these detach themselves and 

 reproduce sexually by means of fertilised eggs ; we thus have alternating asexual and 

 sexual generations. In Myrianida, (also a Syllis) new buds arise on the mother animal 

 even before the hindermost has detached itself. There thus arises a chain of buds of 

 which the hindermost is the oldest and the foremost the youngest. This is a case 

 of axial gemmation which is very similar to strobilation. 



Oligochseta. In autumn Lumbriculus falls into pieces which are all able to 

 regenerate into complete animals. In the genera ^Eolosoma and Ctenodrilus no sexual 

 organs and no sexual reproduction have as yet been observed, only asexual reproduc- 

 tion. In Ctenodrilus monostylos the body becomes constricted in the middle, and finally 

 separates into two pieces, each of which may again divide. These pieces regenerate 

 into normal animals after detachment. It is otherwise in Ct. pardalis. Here, in 

 each segment (except the foremost) behind the dissepiment of the preceding segment, 

 a budding zone appears, in which brain, oesophagus, etc., form. The development of 

 these budding zones takes place from before backward. The segments thus trans- 

 formed finally separate. In each of them the fore-gut and hind-gut become con- 

 nected with the mid -gut and the typical segmentation is developed, so that each 

 becomes a complete individual. In ^Eolosoma, as in the Syllidce, several posterior 

 segments are included in the first bud. While the head forms anteriorly in this bud, 

 it as well as the mother body increases in length, and the latter develops other 

 buds posteriorly before the first and oldest, i.e. the hindermost bud detaches 

 itself. The processes of gemmation are most complicated in Nais and Chwtogastcr, 

 since here in the (anterior) mother individual as well as in the daughter individual 

 new phenomena of gemmation appear before they become detached from each other. 

 Chains of several individuals varying in age and stage of development thus arise. The 

 age and degree of development may be given in a formula. A indicates the fore- 

 most and oldest individual, in which at first the daughter individual B appeared ; 

 then the bud C began to form in the individual B, and so on. An order of develop- 

 ment of buds which has been observed in Nais barbata (from before backward) is as 

 follows : A, F, D, , E, C. Finally, the chain breaks up into its separate parts, which 

 no longer multiply asexually, but increase the number of their segments, and as 

 sexual individuals can develop sexual products. There is therefore here also a kind 

 of alternation of generations, since sexual and asexual reproduction mutually exclude 

 one another. 



XV. Stock Formation. 



The peculiar Syllis ramosa, which lives in deep-sea sponges, forms 

 by means of lateral gemmation much-branched stocks, in which, as in 

 most other Syllidce, special sexual individuals develop and detach 

 themselves. This is the only case of lateral gemmation in the Chcetopoda. 

 Among the Bryozoa, animal stocks of the most various shapes arise 

 by lateral gemmation. They are sometimes tree -like, sometimes 

 tufts, or they may be spread out like webs or crusts ; sometimes 

 many single animals rise from a creeping stem. We thus find repeated 

 in the Bryozoa the forms assumed by the Hydroida. In Loxosoma 

 alone the buds detach themselves, so that a permanent stock is never 

 formed. On the stocks of many Chilostoman Bryozoa peculiar 

 appendages, so-called vibraeularia and avieularia, are found. The 



