Evolution of Fishes from Invertebrates 6i 



an outgrowth and backward growth from the dorsal region 

 of the gullet, so that the mouth and the protrusible probos- 

 cis open by a common aperture. This condition is retained 

 in most species of the nemertean sub-group that Burger 

 has designated the Metanemertini (op. cit. p. 404), and 

 which includes all of the freshwater and land species, as 

 well as related marine ones. But since all of the Nemer- 

 tinea exhibit a common fundamental structure, and as every 

 fact of evolutionary history warrants the possession by them 

 of interconnected details, we will not hesitate to compare 

 details of any species, though laying special emphasis on 

 the Metanemertini. 



In connection with these, and generally treated now 

 as groups of the Chordata, are the Hemichordata or siphon 

 worms, the Urochordata or ascidians, and the Cephalochor- 

 data or lancelets. These will at times be referred to, though 

 the writer would regard all as derivative phyla that have 

 branched off from some advanced nemertean line, or from 

 types intermediate between the nemerteans and the true 

 fishes. 



I. Size and shape. 



Metanemerteans may vary from 3 to 500 mm. in length 

 by y2 to 20 mm. in width. Some species of Drepanophorus 

 are 400 mm. x 20 mm. Dendy says regarding the land 

 form Geonemertes atistralis: "as it lies at rest with the 

 proboscis retracted it has very much the appearance of a slug 

 or of a small planarian worm, and is very soft and slimy. 

 When it begins to crawl, which it readily does on being 

 disturbed, the body elongates, until in large specimens it 

 measures about 40 mm. in length by 2.5 mm. in greatest 

 breadth. The anterior extremity is then seen to be rounded 

 and perhaps slightly swollen into a head, the posterior 

 extremity tapering gradually to a blunt point where the 

 anus is situated." The above two types then are about 

 twenty times longer than broad. 



In size and shape Amphioxiis, Myxine, Petromyzon, 

 and many of the higher fishes show progressive advance 

 on the above, but frequently retain the same relative length 

 to breadth, as well as outline. 



