material for future legs as required. "When the limb is thrown off, the blood-vessels and nerve retract, thus leaving a small cavity in the new-made surface. It is from this cavity that the germ of the future leg springs. A scar forms over the raw surface caused by the separation, which afterwards forms a sheath for the young leg." "As the growth advances, the shape of the new member becomes apparent, and constrictions appear, indicating the position of the articulation; but the whole remains unprotected by any hard covering until the next change of shell, after which it appears in a proper case, being, however, still considerably smaller than the corresponding claw on the opposite side of the body, although equally perfect in all its parts."

*Jones's "General Outline," p. 343.

CHAPTER XXIII.

CRUSTACEA (Crabs and Shrimps).

Continued.

PERHAPS the most singular of all animal existences, and certainly the most remarkable of the Class to which they belong, are those Crustaceans which constitute the Order EPIZOA, so called from their parasitical habits. The grand principle of economy is so perfectly carried out in Creation, that not only is every spot of inorganic nature turned to account in providing for some existences proper to it, but even the bodies of living animals are made to afford a dwelling-place and a feeding-ground for multitudes of other creatures. The intestines, the layers of muscle, the coats of the eye, the sinuses of the skull, afford, as we have already seen,* in different animals, a home for certain creatures of strange conformation, which are found under no other conditions, and are thence called Intestinal Worms, or more correctly ENTOZOA, i. e., animals which live within other animals. The gills of fishes, the breathing pouches, the interior of the mouth, and various parts of the surface of the body, become, on the other

* See Chapter XIII., supra.

hand, the residence of another group, the EPIZOA, of which we are speaking.

Though these two groups of parasitic animals are very diverse in zoological rank, or, in other words, in the degree of complexity which their structure exhibits, they merge into each other by imperceptible gradations, so that there are some intermediate forms (as there almost always are on the confines of great groups), which it is very difficult to arrange in either Class, because this would involve their violent separation from near kindred. It must be borne in mind that our lines of demarcation are artificial, though, for perspicuity's sake, we must draw them somewhere.

One of the most interesting points in the economy of these creatures is the variety which is displayed in their armature. Deprived, for the most part, of limbs, or having these members when present strangely disguised, it was necessary to their existence that they should be furnished with some means of affixing themselves firmly to their prey, and various are the mechanical contrivances which serve this purpose. There is a minute Worm (Gyrodactylus) which lives upon the gills of certain species of the Carp tribe, whose adhering disk, when viewed beneath the microscope, is most formidable to behold. It is armed all around its circumference with sharp curved hooks, while its centre is provided with a pair of much larger hooks, all intended to be plunged deeply into the flesh of the unfortunate fish, while the blood is sucked at leisure. In Caligus, a creature a hundred times as large as that just named, found on various marine fishes, the object is effected by an array of hooked fangs and pincer

like claws, combined with sucking-disks, while a slender tube pierces the flesh and pumps up the vital juices. Chondracanthus,-which looks like a tiny doll dressed up in a long petticoat, fantastically studded all over with curved prickles,-clings to the gills of the John Dory by means of its stout hooked foot-jaws, of which there are three pairs.

More strange still is the furniture of the Lerneopoda. Here two long arms proceed from the thorax, which, curving forward, meet at their tips in front of the head, and uniting, carry a knob or button, which, being thrust into the flesh of some miserable Shark or Salmon, maintains the needful hold for the robber's operations. A similar creature (Achtheres), that infests our common Perch, has a contrivance more elaborate still: the two arms unite as before, but the knob at the point of junction now becomes a bell-shaped cupping-glass, beset within its rim with an array of recurved hooks. Hanging by means of this grappling-iron to the gums of the fish, it allows its body to swing without fear or danger of dislodgment, in spite of the currents that are perpetually flowing through the mouth and gills. In some very long and slender forms constituting the family Penellada, parasitic upon the bodies of fishes, as the Sprat and Anchovy, the entire head is plunged into the tissues of the prey, sometimes into the eye, and is retained there by a curved prong which proceeds backward from each side of the head, exactly on the principle of an anchor. An additional firmness is secured to the Lernea, which infests the gills of the Cod, by the prongs or flukes of the anchor being furnished with processes shooting off irregularly on all sides, which, being

imbedded in the flesh, like the roots of a tree in the soil, resist all opposing influences to drag it away. This form is one of the most bizarre of the whole; for, besides this strange rooting head which is concealed, the exposed body resembles a twisted sausage, without limbs, to which the external ovaries hang, like rolls of thread twined loosely together.

Every ditch and pool is tenanted by tiny nimble creatures, which move through the water by a succession of little leaps, whence they are called Water-fleas. Sometimes the ponds swarm with them to such a degree that the fluid contents seem not water but blood, or other strange liquid, according to the colour of the little animals. These belong to the Order ENTOMOSTRACA, and, though small, many of them being undistinguishable without microscopic aid, they are highly curious. Many of them are enclosed, either wholly or in great measure, in a shell, which takes the form of two convex plates, either soldered together down the back, or united there by a hinge, leaving the opposite edge free for the protrusion of the head and feet. The close resemblance of the latter (Cypridida) to the bivalve shells of MOLLUSCA is remarkable.

In general these little animals have but a single eye in the middle of the forehead, which is of large size in proportion, round, and generally of a brilliant red hue, glistening like a ruby, so that it furnishes a beautiful study under the microscope. In the common Water-flea (Daphnia pulex), the eye is bluish black, evidently composed of about twenty crystalline lenses, and though quite included within the shell, its motions, which are quick and partly rotatory, can be distinctly perceived.