TYPE ANNELIDA. 227 



set of structures formed from it, would give rise to a condition 

 such as is found in the Oligochseta. 



A number of the simpler Oligochaets, in addition to repro- 

 ducing in a sexual manner, also reproduce by division, and in 

 some forms it plays a much more important part than the 

 sexual method, which in jEolosoma is not yet known to occur. 

 In the simplest form of this method of reproduction the 

 animal simply divides at the middle, each portion after sepa- 

 rating regenerating the parts which are wanting. In one spe- 

 cies of Ctenodrilus each metamere except the anterior one 

 may separate and become a new individual ; a phenomenon 

 which might be regarded as illustrating the bud theory of 

 metamerism, but which seems more properly to be a case in 

 which the gradual integration of the multiplied organs has 

 reached its highest development the case standing as the 

 culmination of the process of metamerization rather than as 

 an example of its mode of origin. In Nais a division of the 

 new individuals may begin before they have separated, and 

 chains may thus be produced composed of individuals vary- 

 ing in the stage of regeneration which they have reached, 

 but which eventually separate and may later become sexually 

 mature. 



As might be expected from the occurrence of this mode of 

 reproduction, the power of regeneration of lost parts is pos- 

 sessed in a high degree by the Oligochaeta ; and not only in 

 those forms which habitually reproduce by division, but also 

 in forms like Lumbricus, in which under normal conditions 

 this method of reproduction is unknown. 



Affinities of the Oligocli ceta.- -There is little reason to doubt that the 

 Oligochseta have been derived from the Polychoeta, and represent members 

 of that subclass which have become specially adapted to aquatic or terrestrial 

 modes of life. A few Oligochaats, such as Halodrttu-s, are marine, living 

 below stones between tides; but they are undoubtedly derived from aquatic 

 forms, and cannot be regarded as having any ancestral significance. As re- 

 gards the more definite affinities of the group little can at present be stated 

 with certainty. They have been referred to forms like the Capitellidae, 

 in some of which the parapodia are very much reduced, as is likewise 

 the distinctness of the head, while, as in the Naids, lateral-line sense-organs 

 are present. A more remote relationship through the Archiannelida has 

 also been suggested, but at present no definite evidence is forthcoming as 

 to which view is to be preferred. 



