514 Causes and Course of Organic Evolution 



The remarkable organism Stylochceta fusiformis that was 

 first described as "a new Rotifer" under the name Polyarthra 

 fusiformis (167: 59) is a type that strikingly combines roti- 

 feran, gastrotrichan and chaetophoran characteristics; though 

 most nearly conforming to the second of these groups, as Hlava 

 has shown (168: 247). 



The seven fresh- water species of ^olosoma belonging to 

 the iVphaneura show a ciliated prostomium that corresponds to 

 the trochal disc ia Rotifera (Fig. 21 g, h); a delicate chitinous 

 layer that suggests the similar layer of the latter; set?e that 

 are delicate and embedded in epidermal pouches as in the 

 rotifer C opens, though here they have become numerous and 

 segmentally disposed; a secretion of copious mucus at definite 

 periods in the life history; an alimentary canal that is complete 

 and simple; perfect segmentation indicated only in the anterior 

 area as in some rotifers; nerye ganglia of equally primitive 

 state as in rotifers; simple excretory tubes that show structural 

 similarity to those of Dinophilus and to Rotifera; while as in 

 many of the latter the secreted mucus can be fashioned into 

 a conserving coccoon, inside which the animal can survive 

 adverse environal changes. 



But, apart from Dinophilus, other of the Archiannelida such 

 as Polygordius suggest remarkable agreement with some roti- 

 fers. In Figs. 21 g and 21 i are sho\\Ti the proto-annelidan 

 stage of Polygordius, and a compounded view of Alhertia- 

 Taphrocampa prepared from Hudson and Gosse's beautiful 

 illustrations. 



The Annelida or Chsetophora then we would regard as com- 

 mon descendants with the Platyelmia from fresh-water roti- 

 fers. Very early they seem to have split up into a series that 

 remained fresh-water or in time became land dwellers, such 

 as the aquatic or terrestrial oligocha?tes or "earthworms"; 

 also into a group that migrated shorewards and there became 

 the polychsete or marine worms. Such migration may have 

 taken place while the migrants were still more nearly rotiferan 

 than annelidan in structural detail. But the few connecting 

 types now left to us make more exact decision impossible. 



That even the highly modified and greatly more complex 

 polycha^te Nereis is largely molded on a rotiferan basis is abun- 

 dantly shown by comparison. For in both we note the soft 

 flexible dorsiventral body, the pryeoral region that in the larval 



