If you look at the head of a crab, a lobster, or a prawn, you will see that it is furnished with jointed antennæ, like that of insects; but whereas in insects there is never more than a single pair, in the creatures of which I am speaking there are two pairs. In the prawn you may suppose, at first sight, that there are four pairs; but that is because the internal antennæ terminate each in three many-jointed bristles, in structure and appearance exactly like the bristles of the outer pair, two of the three being nearly as long as the outer, while the third is short. In the lobster, the internal are two-bristled, both bristles rather short, while the external are very long. In the flat-crabs each pair is simple, the inner minute, the outer long. In the great eatable crab each pair is very small, and they are dissimilar. [Illustration: Two Strange Zoophytes 1, _Holtenia Carpenteri_, a species of glass sponge; 2, _Pentacrinus Asteria_, the only survivor of the Crinoid family] Now taking the last-named animal as the representative of his class, let us examine one of his inner antennæ first. It consists of a jointed stem and a terminating bristle; the latter furnished with small hairs common to the general surface of the body, and with long, delicate, membraneous filaments (_setæ_), often improperly called _cilia_, which are larger and much more delicate in structure than the ordinary hairs. The basal joint is greatly enlarged: if it be carefully removed from its connection with the head, and broken open, it will be found to inclose in its cavity a still smaller chamber, with calcareous walls of a much more delicate character than the outer walls. This internal shell is considered by Mr. Spence Bate to be a _cochlea_, from its analogy, both in structure and supposed use, to the organ so named in the internal ear of man and other vertebrate animals. It is situated, as has been said, in the cavity of the basal joint of the internal antennæ, and is attached to the interior surface of its wall furthest from the median line of the crab. It has a tendency to a spiral form, but does not pass beyond the limits of a single convolution. If this interior cell does indeed represent the _cochlea_ of more highly constructed ears--to which it bears some resemblance, both in form and structure--then it seems to identify, beyond dispute, these inner or upper antennæ as the organs of hearing. Now with this conclusion agrees well the manner in which the living animal makes use of the organs in question. The crab always carries them erect and elevated; and is incessantly striking the water with them, with a very peculiar jerking action, now and then vibrating, and, as it has been called, “twiddling” them. These antennæ, therefore, appear to be always on the watch: let the animal be at rest, let it be feeding, no matter, the superior antennæ are ever elevated and on constant guard.