THE ANNELIDA 



5 1 



make definite assertions about the origin of the segmented 

 bodies of worms, and of their most characteristic organs. 

 Thus we always find the mesoblast formed from a pair of cells 

 called mesomeres, which are set apart during the segmentation 

 of the ovum, and give rise to a pair of cellular bands placed 

 right and left of the middle line. 



We can assert, as a general rule, that the metamerism of the 

 adult worm is brought about by the segmentation of these 

 bands, and that the ccelom arises as a series of paired pouches 

 through the hollowing out of the segments thus produced. We 

 can assert that the true nephridia are formed from the ecto- 

 derm, and thence grow towards and open into the coelomic 

 pouches, whilst peritoneal funnels functioning as generative 

 ducts are formed as outgrowths from the ccelom towards the 

 exterior. We can assert that the head-cavity or prostomium 

 differs from the paired ccelomic cavities of the body proper, and 

 is in fact the representative of the provisional body-cavity of 

 the trochosphere larva, and therefore a part of the persistent 

 blastoccele. We can assert that the supra-oesophageal ganglion 

 is formed from the apical sense-organ of the trochosphere, and 

 that the ventral cord primitively arose independently of it, as 

 a differentiation of the ventral epiblast forming a thickening 

 which may be called the ventral plate. And, finally, we may 

 fairly assume that the multi-segmented Annelid was evolved 

 from an ancestor which showed no more trace of segmentation 

 than the trochosphere itself. 



Having satisfied ourselves on these points, we may naturally 

 ask : What is the significance of this metameric repetition of parts 

 which is so prominent a feature in the development and adult 

 anatomy of Annelid worms ? What explanation can we give of 

 this peculiar course of evolution resulting in an animal which 

 may be described as consisting of a head-region (prostomium 

 and mouth segment), to which a string of segments is attached ? 

 It is, of course, difficult, if not impossible, to give an exact 

 answer to such a question, but the comparison of the adult 

 anatomy of a number of worms enables us to make a plausible 

 conjecture. It has been shown that in Polygordius nearly all 

 the somites contain gonads, that in the Polychasta the genera- 

 tive cells, ova or spermatozoa, may originate from nearly any part 

 of the ocelomic epithelium, and that the generative products, 

 ova and spermatozoa, are invariably derived from that epi- 



