in structure to that of the
_Eucleptella_; numerous crystalline needles protrude from the surface
of the upper part. Lately some specimens of _Holtenia_ have been found
on the coast of Florida.

[Illustration: Fig. 3.--HOLTENIA CARPENTERIA.]

Glass sponges serve as dwellings for numerous animals, especially
crustaceae. A small shrimp inhabits the tubes of the _Eucleptella_, a
male and a female generally living together. They are shut up as in a
prison in their crystalline home, as they are generally too large to
pass through the meshes formed by the bundles of crystals. It was
formerly believed that these skeletons had actually been built by the
shrimps, and we can find no explanation for this curious circumstance,
other than that the shrimps entered these habitations while very small
and became too large to leave them.

* * * * *

PLANTS PROTECTED BY INSECTS.

Mr. Francis Darwin, in a lecture on "Means of Self-Defense among
Plants," delivered lately at the London Institution, said that one of
the most curious forms of defense known is afforded by a recently
discovered class of plants, which, being stingless themselves, are
protected by stinging ants, which make their home in the plant and
defend it against its enemies. Of these the most remarkable is the
bull's-horn acacia (described by the late Mr. Belt in his book "The
Naturalist in Nicaragua"), a shrubby tree with gigantic curved thorns,
from which its name is derived. These horns are hollow and tenanted by
ants, which bore a hole in them, and the workers may be seen running
about over the green leaves. If a branch is shaken the ants swarm out
of the thorns and attack the aggressor with their stings. Their chief
service to the plant consists in defending it against leaf-cutting
ants, which are the great enemy of all vegetation in that part of
America. The latter form large underground nests, and their work of
destruction consists in gathering leaves, which they strip to form
heaps of material, which become covered over with a delicate white
fungus, on which the larvae of the ants are fed, so that literally they
are a colony of mushroom growers. The special province of the little
stinging ants, which live in the thorns of the acacia, is, therefore,
to protect the leaves of the shrub from being used by the leaf-cutters
to make mushroom beds. Certain varieties of the orange tree have
leaves which are distasteful to the leaf