Dec., 1892. 
MICROSCOPIC POND-LIFE. 
267 
genus, known as Vaginicola, have skeletons, • although most 
of the family are without them. In these two families— 
Monads and Vorticellidse—the cell is the unit, for the animal 
consists of only one cell. Yet they have a way of living in 
“colonies,” which suggests the Metazoa, or animals whose 
bodies are made up of many cells. Sponges are very 
interesting and puzzling things to the Zoologist who makes 
classification his hobby. Is a sponge “ an animal ”—one of 
the Metazoa ?—or does it belong to the Protozoa ?—in which 
case it is not “ an animal” but is only a huge “ colony” of 
unicellular “ animals.” There is a good deal to be said on 
both sides. In its canals may be found organisms identical 
with the collared monads, which are apparently indivisible 
from the animal. This goes to show that it is merely an 
overgrown colony of monads. But it has a skeleton of 
spicules not unlike the spicules of gorgonia, and, in the 
autumn, it produces a kind of “ winter egg,” which has a hard 
shell. This makes one think it “ an animal ” and belonging 
to the Metazoa. And, in favour of this view, I would suggest 
that the “ collared monads ” in the canals, which no sponge 
is ever found without (if you look for them in the right way), 
may really be individual monads living in partnership with 
the higher animal—a case of “ symbiosis,” in fact. 
Passing from the Protozoa to the Metazoa, let us take for 
example the common Hydras. Those found in most of our 
rivers are without the vestige of a skeleton, but there is a 
genus first found in brackish water, and which is very abun¬ 
dant in a perfectly freshwater canal at Chester, which has a 
hard case or skeleton, like the marine relatives of the Hvdra. 
This is called Cordylophora, and it is a very picturesque little 
creature, with the usual sting-cells in its skin. Most micro- 
scopists have heard of and seen these sting-cells. Few know 
how they act. The little sting is coiled up inside the cell, 
something like the hair-spring of a watch. The common 
idea is that the animal uncoils the stings when it likes, 
and pierces its prey with them, the stings remaining attached 
to its arms. That is a mistake. There is a kind of trigger 
projecting from the skin a little way. When this is pressed 
by the arm of the animal squeezing its prey it releases the 
spring, which uncoils “like a flash” and becomes per¬ 
fectly straight and rigid. The sting, which has a little round 
head to it, comes right out of the Hydra, and sticks into its 
prey like a tiny pin, with the head outermost, of course. 
Hundreds of these atomic pins are shot out by the Hydra 
against (say) a water flea, and by the time it is dead, the 
poor little Daplmia looks like a round pincushion full of pins. 
