MULTICELLULAR ORGANISMS, 23 
be fitly undertaken at this stage. The Polyps are easily obtain- 
able from ponds in which they are found attached to various 
kinds of weeds. To the naked eye, they resemble translucent 
masses of jelly with a greenish or reddish tinge. They range 
in size from one quarter to one half an inch; are of an elongated 
cylindrical form ; provided at the oral extremity with thread- 
like tentacles of considerable length, which are slowly moved 
about in all directions; but they and the entire body may short- 
en rapidly into a globular mass. They are usually attached at 
the opposite (aboral) pole to some object, but may float free, or 
slowly crawl from place to place. It may be observed, under 
the microscope, that the tentacles now and then embrace some 
living object, convey it toward an opening (mouth) near their 
base, from which, from time to time, refuse material is cast out. 
It may be noticed, too, that a living object within the touch of 
these tentacles soon loses the power to struggle, which is owing 
to the peculiar cells (nettle-cells, urticating capsules, nemato- 
cysts) with which they are abundantly provided, and which se- 
crete a poisonous fluid that paralyzes prey. 
The mouth leads into a simple cavity (celom) in which 
digestion proceeds. The green color in Hydra viridis, and the 
red color of Hydra fusca, is owing to the presence of chlorophyl, 
the function of which isnot known. Hydra is structurally a 
sac, made up of two layers of cells, an outer (ectoderm) and 
an inner (endoderm); the tentacles being repetitions of the 
structure of the main body of the animal, and so hollow and 
composed of two cell layers. Speaking generally, the outer 
layer is devoted to obtaining information of the surroundings; 
the inner to the work of preparing nutriment, and probably, 
also, discharging waste matters, in which latter assistance is 
also received from the outer layer. As digestion takes place 
largely within the cells themselves, or is intracellular, we are 
reminded of Vorticella and still more of Amoeba. There is in 
Hydra a general advance in development, but not very much in- 
dividual cell specialization. That of the urticating capsules is 
one of the best examples of such specialization in this creature. 
A Polyp is like a colony of Amoebe in which some division of 
labor (function) has taken place; a sort of biological state in 
which every individual is nearly equal to his neighbor, but 
somewhat more advanced than those neighbors not members of 
the organization. 
But in one respect the Polyps show an enormous advance. 
Ordinarily when nourishment is abundant hydra multiplies by 
