152 THE HISTORY OF CREATION. 



worms (Rotifera). The unknown, extinct, primary forms 

 of tlie tribe of Sea-stars (Echinoderma), and. of the tribe 

 of the articulated animals (Ai'thropoda), were nearest akin 

 to the Ring-worms. On the other hand, we must probably 

 look for the primary forms of the great tribe of Molluscs in 

 extinct Worms, which were very closely related to the 

 Moss-polyps (Bryozoa) of the present day; and for the 

 primary forms of the Vertebrata in the unknown Coelomati, 

 whose nearest kin of the present day are the Sea-sacs, 

 especially the Ascidia. 



The class of Sea-sacs (Tunicata) is one of the most 

 remarkable among Worms. They all live in the ocean, 

 where some of the Ascidise adhere to the bottom, while 

 others (the sea-barrels, or Thaliacea) swim about freely. In 

 all of them the non-jointed body has the form of a simple 

 barrel-shaped sack, which is surrounded by a thick cartila- 

 ginous mantle. This mantle consists of the same non- 

 nitrogenous combination of carbon, which, under the name 

 of cellulose, plays an important part in the Vegetable King- 

 dom, and forms the largest portion of vegetable cellular 

 membranes, and consequently also the gTeater part of wood. 

 The barrel-shaped body generally possesses no external ap- 

 pendages. No one would recognise in them a trace of rela- 

 tionship to the highly differentiated vertebrate animals. 

 And yet this can no longer be doubted, since Kowalewsky's 

 investigations, which in the year 1867 suddenly tlu-ew an 

 exceedingly surprising and unmistakable light upon them. 

 From these investigations it has become clear that the indi- 

 vidual development of the adherent simple Ascidian Phallusia 

 agrees in most points with that of the lowest vertebrate 

 animal, namely, the Lancelet (Amphioxus lanceolatus). 



