Evolution of Animals 401 



and the Crustacea, they were ahiiost surely derived from an 

 ancient cambrian or late archaean ancestry, of rotiferan 

 structure and lacustrine habit. The writer has not attempted 

 to estimate the number of living genera, which after all, in 

 the present group, is of minor import when one has regard 

 to their varied structural ramifications. For sake of com- 

 parison he has roughly included 3500 genera, most of which 

 are truly terrestrial, a relatively small number are now fresh- 

 water, wliile the very few littoral or semimarine types may 

 well be neglected. 



The Hemichordata is a small group that we would view 

 as marine derivatives from a form intermediate between liigher 

 marine Nemertinea and the Cephalochordata. The almost 

 world-wide distribution of Balanoglossus suggests an ancient 

 ancestry for the group, that now consists of 11 genera and 

 about 32 species. 



The Ascidiacea or Urochordata is also a marine series, which 

 during the past quarter century has been shown to be rich In 

 genera and species. In all 130 genera at least are recognized. 

 The above two groups are further referred to in a succeeding 

 chapter (pp. 537-540). 



The simplest living chordate or vertebrate genus Amphioxus 

 or Branchiostoma inhabits sandy seashores, where it burrows 

 readily. But as stated later we would regard it as a divergent, 

 and rather widely divergent, type from the main line of inver- 

 tebrate-vertebrate ascent. As now known the group includes 

 4 marine genera. 



The Cyclostomata or Marsipobranchii may be said to con- 

 sist of 5 well marked genera, Myxine, Bdellodoma, Petronujzon, 

 Mordacia, and Geotria. These for the writer have a special 

 interest, as being, according to his views, in almost direct 

 continuity with the main vertebrate line of evolution (p. 418). 

 Bearing in mind that such genera of the Nemertinea as Geo- 

 nemertes, Tetrastemma, and Stichostemma are wholly or chiefly 

 land or fresh-water forms, even though the large majority 

 are marine, if we build up a compounded type of nemertean 

 from these, the resulting structure would very largely conform 



