44 



Returning to the main stem of the table, we find a 

 number of comparatively small groups, lying between the 

 primitive Echinoderms and the higher worms, which are 

 probably all derived from side branches given off from what 

 may be called the middle third of the vermean axis — the 

 lower third being the region below the point of origin of the 

 Mollusca, and the upper third above the origin of the 

 Arthropoda. These small groups are all rather aberrant, 

 and some of them are certainly degraded. 



The Nematelmian worms are represented by a branch 

 extending upwards to the right above the common origin of 

 the Chordata and the Echinodermata. The members of this 

 group are less unlike the typical Vermes than is the case with 

 most of the other branches from the middle third of the 

 vermean axis. The highest forms of the group are the 

 Nematoda, while the Acanthocephala (Echinorhynchus) must 

 be regarded as a degraded offshoot, sloping downwards from 

 near the base of the Nematelmian branch. The degenera- 

 tion of Echinorhynchus, and probably of some of the 

 Nematoda also, is a result of their parasitic mode of life. 



The Gephyrea have probably arisen from the main stem, 

 not far from the point of origin of the Nematelmia. They 

 have developed a spacious coelom, and in most cases a long 

 convoluted alimentary canal. On the other hand, they retain 

 the characters of primitive Vermes in the absence of true 

 segmentation, and in the larval prae-oral lobe developing into 

 an important part of the front of the body. In most, a small 

 number of paired nephridia are present, and in some cases 

 (e.g,y Bonellia), two of these placed at the posterior end of 

 the body become greatly enlarged to form branched organs, 

 placing the coelom in communication with the proctodaeum, 

 as in the case of the respiratory trees in the Holothuroidea. 



The affinities of the Brachiopoda are still unsettled. The 

 larvae of some forms {e.g., Argiope) appear to shew close 



