The Zoophytes. 303 
portion, where the central cavity of each individual is 
continuous throughout with that of every other form, the 
nutrient fluid elaborated from the food material by any 
one individual, mixes throughout the whole system for 
the benefit of all ; but this, though it insures a circulation 
of the common fluid throughout the colony, is by no 
means comparable to the distinct circulatory system of 
any of the higher types. 
In very many of these colonial forms, a most wonderful 
specialisation of function, accompanied by a corres- 
ponding modification in structure, has taken place among 
the various individuals of the colony, by which certain 
buds or individual forms, comparable to a battery of 
stinging cells, perform the functions of grasping organs, 
being thus set apart for defending the colony and for 
seizing food ; while other forms, of much larger size, 
receive the food thus provided, and digest it for the 
common good — an adaptation to a division of labour, 
that, in relation to the lowly organised forms amongst 
which it obtains, is perhaps unequalled even by the 
complex associations of human civilisation. 
Though the most characteristic forms of the Zoophytes 
are those that take on the colonial habit — a habit due to 
the continuous budding or branching of the parent or 
individual organisms to form other beings which remain 
attached to each other — yet a considerable number of 
forms exist which lead a simple life and never give rise 
to colonies ; and these simple organisms are to be found 
in each of the three groups, the Hydroids, the Sponges 
and the Corals, into which the Zoophytes are divided. 
The little Hydra or Fresh-water Polyp, a form that 
presents the typical bag-like body with arm-like feelers 
