THE MAIN LINES OF ANIMAL EVOLUTION 



Figure 49. Peripatus. (Courtesy Ward's Natural Science Establishment.) 



of a hemocoel and especially because of the tracheal system, they are now 

 generally treated as a class of the Arthropoda. But these are not the only 

 possibilities. They are sometimes treated as an independent phylum inter- 

 mediate between the Annelida and the Arthropoda. And Light has urged 

 that, as they are not properly separable from either of the major phyla, 

 the entire annelid-onychophoran-arthropod series ought to be recognized 

 as one great phylum under the name Articulata, With no other series of 

 phyla are such considerations possible and hence the statement with 

 which this discussion began, that the origin of the Arthropoda from the 

 Annelida is more certain than the origin of any other phylum. It is an 

 unfortunate fact that the Onychophora are scantily known in the fossil 

 record. 



Minor Protostomes. A few minor phyla also show the general char- 

 acters of the protostome line. Two of these, the Sipunculoidea and the 

 Echiuroidea, are worm-like burrowers of the tide flats. They are generally 

 treated as minor annelids, an arrangement which is more convenient than 

 accurate. Yet they probably are more closely related to the Annelida than 

 to any other phylum. The three remaining phyla, the Bryozoa, the Pho- 

 ronida, and the Brachiopoda, are more difficult to place. Like the Ento- 

 procta, they feed by means of a lophophore, but unlike that phylum they 

 have a coelom. The Bryozoa are small, colonial animals which superficially 

 resemble the Entoprocta, but actually differ from them fundamentally. 

 The Phoronida includes only two genera. The animals are elongate, worm- 

 like creatures which dwell in tubes on tide flats. When the tide is in, the 

 lophophore projects into the water to feed on plankton and detritus. The 



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