SOME PROBLEMS OF ANNELID MORPHOLOGY. 63 



said, appears to be budded off from the head. How ? 

 By the successive formation of a series of somites, each 

 of which contains its own segment of the ahmentary 

 canal and of the circulatory apparatus, a pair of excre- 

 tory organs, a pair of ganglia and nerves, and in some 

 cases gills, locomotor organs, sense-organs — eyes, it 

 may be — tactile organs, and the like. Each somite 

 has a complete, or nearly complete vital apparatus of 

 its own ; and in some annelids (though these cases are 

 rare) the somites may become separate, lead indepen- 

 dent lives, and develop finally into complete individuals 

 like the original worm. 



These facts irresistibly suggest the question : is not 

 the trunk to be regarded as a linear colony of sexual in- 

 dividuals, successively budded off from the asexual 

 head .'^ — 2^^^cisely as sexual medusae are budded off from 

 the asexual scyphistoma, or as proglottides are seg- 

 mented off from the scolex of a cestode worm. If this 

 question be answered in the affirmative, then all metam- 

 eric animals, vertebrates and man included, must be 

 colonial organisms, comparable, in point of individuality, 

 with a hydroid or polyp colony. A number of eminent 

 zoologists do not hesitate to accept this conclusion. 

 One of the latest and best students of annelid develop- 

 ment (Kleinenberg) regards the trochophore as being 

 simply a modified asexual medusa ( ! ). He finds in it 

 the characteristic medusan nerve-ring, the modified 

 velum (prototroch), the umbrellar and sub-umbrellar 

 regions. He does not state definitely his conception 

 of metamerism, which we are left to infer ; but other 

 morphologists have not hesitated to interpret his views 

 in accordance with the colonial theory. 



