COLONIAL ANIMALS 101 
“collar” from the centre of which springs a thread (flagellum) 
which executes lashing movements. Within the jelly are other 
amceba-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). 
CotoniaL SponcEs (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 
(Halichondria panicea), which may be seen as an encrustation 
of light-brown colour on rocks near low-tide mark. 
CotontaL ZOOPHYTES (C@LENTERATA), —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 
