36 ANIMAL FORMS 
essentially the same structure but with various modes of 
branching (for some of the commoner modes, see Fig. 17). 
In some of the higher forms a division of labor has arisen 
among various members of the association which has led to 
most interesting results. For example, Fig. 19 represents 
a species of hydroid found investing the shells of sea-snails 
occupied by hermit crabs (Fig. 60). To the unaided eye 
its appearance is that of a delicate vegetable growth, but 
when placed under the microscope it is found to consist of 
a multitude of Hydra-like 
animals united by a hollow 
branching root system con- 
necting the gastric cavities 
of all of them (Fig. 19). 
Certain individuals (a) 
with tentacles and a mouth 
resemble a Hydra; others, 
without a mouth and ten- 
tacles, are reduced to a 
Fie. 19.—An enlarged portion of a hydroid club-like form (2) liberally 
colony (dydractinia), showing (a) the supplied with nettle-cells 
nutritive polyp, (0) the defensive polyp, upon their free extremi- 
and (c) the reproductive polyp. ties ; alia the “ain type 
(c), likewise devoid of a mouth, possesses rudiments of ten- 
tacles below which are borne numerous clumps of repro- 
ductive cells. The first type, the only one possessing a 
mouth, captures the food, and after digesting it distributes 
the greater portion to the remaining members by means of 
the connecting root system; those of the second form, de- 
fending the others by means of their nettle-cells against 
the inroads of a foreign enemy, are the soldiers of the colo- 
ny; while the third type produces the eggs from which 
new individuals develop. 
In some of the higher Hydrozoa, the Portuguese man- 
of-war (Fig. 20), this division of labor has reached a more 
advanced stage of development, and in addition the entire 
