An individual of Eschara cervicornis, highly magnified.
a. Tentacula
b. First digestive cavity.
d. Stomach.
f. Anus.
Retepora cellulosa. (Neptune's Ruffle.)
The Lepraliæ, or Sea-Scurfs, form thin calcareous crusts of a white-yellow or reddish colour on rocks, shells, and sea-weeds. To the naked eye they appear as rude unsightly eruptions, so as to justify their name derived from the hideous leprosy of the East, but, when magnified, their cells, generally disposed in regular concentric rows, exhibit a surprising diversity and elegance of structure. Forty species are found in the North Sea alone; hence we may judge how great the number of still unknown forms must be that spread their microscopic traceries over the algæ and shells of every zone.
It would lead me too far were I minutely to describe the Cellulariæ with their cells disposed in alternating rows on narrow bifurcated branches; the Tubulipores, with their mouths at the termination of tubular cells without any movable appendage or lip; the Bowerbankias and Lagunculas, with their creeping stems and separate cells; suffice it to say that a wonderful exuberance of fancy displays itself in the structure of the numerous varieties of the Polyzoa.
A. Portion of a Cellularia, magnified.
B. A Bird's Head Process, more highly magnified, and seen in the act of grasping another.
But a closer inspection reveals still greater miracles to the marine microscopist, for most genera, and chiefly the Cellulariæ, possess very remarkable appendages, or processes, presenting the most striking resemblance to the head of a bird. Each of these processes, or "aviculariæ," as they have been named, has two "mandibles," of which one is fixed like the upper jaw of a bird, the other movable like its lower jaw; the latter is opened and closed by two sets of muscles, which are seen in the interior of the head, and between them is a peculiar body, furnished with a pencil of bristles, which is probably a tactile organ, being brought forwards when the mouth is open, so that the bristles project beyond it, and being drawn back when the mandible closes. During the life of the polyzoon, these tiny "vulture-heads," which are either sessile or pedunculated, keep up a continual motion, and it is most amusing to see them see-sawing and snapping and opening their jaws, and then sometimes in their incessant activity even closing upon the beaks of their neighbours.
It is still very doubtful what is their precise function in the economy of the animal; whether it is to retain within reach of the ciliary current bodies that may serve as food, or whether it is like the pedicellariæ of the sea-urchins to remove extraneous particles that may be in contact with the surface of the polyzoary. The latter would seem to be the function of the "vibracula," which are likewise pretty generally distributed among the polyzoa. Each of these long bristle-shaped organs, springing at its base out of a sort of cup, that contains muscles by which it is kept in almost constant motion, sweeps slowly and carefully over the surface of the polyzoary, and removes what might be injurious to the delicate inhabitants of the cells, when their tentacles are protruded. So carefully have these lowly molluscs been provided for!