COLONIAL ANIMALS 105 
ciliary action into the mouths of the unmodified members of the 
colony, thus serving as food. But there is no definite proof that 
such is the case. Individual Moss-Polypes may also undergo still 
greater modification into whip-like threads that actively lash about 
in all directions. Cleanliness and defence have here again been 
suggested as the ends to be served, and cases 
have been observed where the action is so vig- 
orous as to move the entire colony about. That 
the surrounding water should be thoroughly 
stirred up is probably advantageous with refer- 
ence both to feeding and breathing. The only 
thing, however, that we definitely know about 
these curious structures is, that they have been 
evolved from bird’s-head individuals by suppres- 
sion of the “head”, and prolongation of the 
“lower jaw ” into a slender filament. 
CotontaL Tunicates (UrocHorpa).—The 
formation of colonies is clearly related to powers 
of increase by means of budding or fission, and 
consequently all the members of certain animal 
groups devoid of such powers, e.g. Arthropods 
and Molluscs, are non-colonial. This is also true 
for the vast majority of Backboned Animals, 
the most notable exception being afforded by 
many species of the lowly and degenerate forms 
known as Sea-Squirts, Tunicates, or Ascidians. 
Most of these are fixed to some firm object when _ Fig. 1094.—Parts of Col- 
adult, and their sedentary life has no doubt had i’ pay na 3 
much to do with the degeneration they have ieee a perenes 
undergone (see vol. iii, p. 421). A good many red m lemscssof a0 or 
Tunicates are non-colonial or “solitary”, but he i re 
others bud to produce colonies of various shape. 
In such species the individual members may be borne on a creep- 
ing stem and clearly marked off from one another, much as in a 
hydroid zoophyte, or the association may be much more intimate. 
In the latter case the individuals are sunk within a sort of common 
body (like the ccenosarc of colonial corals), and there is a contin- 
uous protective investment or common test. A good instance is 
afforded by Botryllus (fig. 1095), to be found at low tide on our 
coasts as a sort of bluish encrustation on sea-weeds and stones. It 
