332 A. T. MASTERMAN. 



of the metaraerically segmented Iii vertebrata^ the Annelids 

 and Arthropoda. 



One cannot here enter further into these ideas, but they may 

 be appropriately expressed by the accompanying tree. 



Though it would be difficult to imagine a more rudimentary 

 condition of the notochord than that of Actinotrocha^ the 

 ontogeny of the Echinodermata and Brachiopoda^ or even 

 the Sipunculoidea and Polyzoa, may yet furnish interest- 

 ing rudiments of chordate organs. 



Another suggestive line of thought is the gradual abolition 

 of the ar chimeric segmentation and elaboration of the 

 metamericin the Annelid a, Arthropoda, and Eu-chor da. 

 The nerve-ganglion ofActinotrocha is probably homologous 

 with the trochophoral apical ganglion, and hence with the 

 supra-oesophageal ganglion of the Annelida. 



The protomere is most likely represented by the prosto- 

 mium, the mesomeres by the peristomium, and the post-oral 

 or mesomeric nerve-ring by the circum-oesophageal ring. The 

 mesomeric coelomic cavities may possibly break down, leaving 

 the trocophoral head-kidneys as mesomeric nephridia, and 

 . there are indications that all the coelomic cavities (and appen- 

 dages in Arthropoda) are post-oral, probably metameric.^ 



It is not difficult to understand why the metamerically seg- 

 mented animals iiave replaced, in nature's highways, the 

 Archicoelomata, retaining only the archimeric segmenta- 

 tion into three parts. 



Just as a limit is placed to the organic capabilities of single- 

 celled individuals, which is easily surpassed by the Metazoa, 

 so the animals starting with a greater number of segments, 



* " It appears that the antennse ofPeripatus and the antennse of Myriapods 

 and Hexapods, as well as the two pairs present in Crustacea, are all of them 



post-oral parapodia shifted in relation to the mouth The conclusion 



to which embryological study (as to segments of body-cavity, so-called cere- 

 bral ganglion, and position of early rudiments of antennary appendages of 

 Arthropods) tends is that in the various branches of Arthropod descent .... 

 there have been forward movements or shiftings of post-oral segments and 

 their parapodia." — E. Ray Lankester, 'Natural Science,' April, 1897, p. 265. 



