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. 



COLONIAL TUNICATES (UROCHORDA). 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 



11 11* i 1 / i 1111 



adult, and their sedentary life has no doubt had 

 much to do with the degeneration they have 

 undergone (see vol. iii, p. 421). A good many 

 Tunicates are non- colonial or "solitary", but poceii 



f whi- 



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 



Fi g . ioo 4 .-parts of coi- 



onies of Moss -Polypes, en- 



i a r g ed. A, Bu gu i a ; B , B , B , 



showing three 



hip-shaped individuals. 



