152 THE HISTORY OF CREATION. 
worms (Rotifera). The unknown, extinct, primary forms 
of the tribe of Sea-stars (Echinoderma), and of the tribe 
of the articulated animals (Arthropoda), 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 Ccelomati, 
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 Ascidiz 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 greater 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 threw an 
exceedingly surprising and unmistakable light upon them. 
From these investigations it has become clear that the indi- 
vidual deveiopment of the adherent simple Ascidian Phallusia 
agrees in most points with that of the lowest vertebrate 
animal, namely, the lLancelet (Amphioxus lanceolatus). 
