COLONIAL ANIMALS ' , ; & \ t . \ :' '-. { '. \6 i> 



"collar" from the centre of which springs a thread (flagellum) 

 which executes lashing movements. Within the jelly are other 

 amoeba-like individuals, which divide actively, some of the pro- 

 ducts of division serving to increase the size of the colony, while 

 others are probably liberated to found fresh communities. Pro- 

 terospongia is of special interest, as it suggests the way in which 

 Sponges have possibly been evolved from simpler animals, for 

 it is characteristic of Sponges that the spaces within their bodies 

 should be more or less lined with "collar cells" that strikingly 

 resemble the collar-provided individuals of the colonial Animalcule 

 just described. 



All forms higher in the scale than the Protozoa are collectively 

 known as Many-celled Animals or Metazoa. In any one of these, 

 e.g. a Zoophyte, a Worm, or a Mollusc, the body is a more or 

 less complex community of cells, exemplifying in various degree 

 the principle of division of physiological labour, with accom- 

 panying specialization. And there can be little doubt that these 

 cell-communities have been gradually evolved from colonial Pro- 

 tozoa. This view has been discussed to some extent in an 

 earlier section (see vol. iii, p. 333). 



COLONIAL SPONGES (PORIFERA). In this group of animals 

 the colonial condition is the rule, a colony being produced by the 

 budding or incomplete fission of an original individual. Some- 

 times the members of a community are fairly distinct (see 

 vol. iii, p. 343), but in other cases it is difficult or even impossible 

 to say where one ends and others begin. The absence of sharp 

 boundary-lines between adjacent individuals is well exemplified 

 by a very common British species, the Crumb-of-bread Sponge 

 (Halickondria panicea), which may be seen as an encrustation 

 of light-brown colour on rocks near low-tide mark. 



COLONIAL ZOOPHYTES (CCELENTERATA). Vegetative propaga- 

 tion by means of budding or fission is very characteristic of 

 members of this large group, and the buds or fission-products 

 commonly remain united together to form colonies, of which the 

 members are usually clearly marked off from one another. They 

 are united together by what may be termed a " common flesh " 

 (ccenosarc), and their digestive cavities all communicate with a 

 more or less complex system of canals by which this is traversed. 

 It therefore follows that food taken in and digested by one in- 

 dividual may benefit other members of the same community, a 



