If we see many of these crustaceans live a joyous life while young, there are others which seem to practise economy, and to emancipate themselves when they have grown old. Mons. Hesse and Mr. Spence Bate a few years since revealed the secrets of their existence.
Naturalists had recognized some crustaceans under the name of Ancei, and others under the name of Pranizæ, living together upon fishes, but with very different organs for fishing and swimming. M. Hesse, curious to know the manner of life of the Pranizæ, made observations on them in a small aquarium, and he perceived that the parts of the mouth were all at once transformed into formidable mandibles, which caused them to resemble Ancei. As it had often occurred with respect to other groups, that the same crustacean at different periods of its evolution had been taken for different animals, the naturalist of Brest had some suspicion as to their identity, and soon ascertained by direct observation that he had not been mistaken. The Pranizæ become Ancei, and live upon fishes under their first form, like caligi and arguli. Nothing can be seen which is more curious than these crustaceans, which ride on the back or the sides of fishes, and assume there every possible attitude.
The Pranizæ fix themselves in the mouth and in the gills as well as on the skin. Some are found on sharks as well as on osseous fishes. They fear neither heat nor light, and do very well under damp sea-weed while waiting for the return of the tide. They run and swim with the same facility. When in the condition of Ancei, they lose their agility, and, under this form, all denotes their sedentary habits. They appear to live in holes, at the bottom of which they defend themselves with their powerful mandibles. It has been observed that fecundation is accomplished, as in the Axolotls, before the evolution is complete, but that the eggs are not laid until the animal assumes the form of Anceus.
We may here remark that the change of appearance takes place only among the females; the males preserve their dress and their liberty. Some naturalists assert that we must not accept the metamorphosis of either sex as an established fact, except for the purpose of arrangement. All, however, tends to show that Mons. Hesse has fairly interpreted facts; but it appears to us probable that the whole of the history of these strange crustaceans is not fully known.
Fishermen have long since known whale-lice, the Cyami of naturalists, of which we have already made mention while speaking of free messmates. They live at liberty on the skin of their host, and multiply with extreme rapidity. These Cyami have a regular form, but completely different from the others, and have given (like the Ricini and the afore-mentioned crustaceans), great trouble to systematic zoologists. The place which they ought to occupy is far from being definitely fixed. At all events they may be considered as a shorter kind of Caprellæ.
As each whale has cirrhipedes which are peculiar to itself, so each has its own cyami. Professor Lütken, of Copenhagen, has made known ten or twelve species, all found on cetacea, in the two hemispheres. The supposed Cyamus, represented by Dr. Monedero as living on the Biscayan whale, is a Pycnogonon.
The Anilocræ and the Nerocilæ, like the Cyami and other genera, establish themselves on the back of a fish which is a good swimmer. Jealous of their liberty, they preserve their oars and their fins, in order to change their convoy, when the desire seizes on them, and do not imitate the Bopyrians, which instal themselves
on the narrow branchial cavity of some decapod crustacean, and as soon as they have entered, throw off all their travelling baggage; in fact, there is no other means for them to gain admission; their lot is identified with that of their host; they can no longer live without him. The female only, it is true, thus renounces her liberty; she sacrifices herself, as usual, for her family, while the male, far from giving himself up, preserves his defensive arms, his claws, and his liberty.
The crustaceans called Caprellæ are perhaps not so independent as they appear to be; it is not impossible that their place may be among the crustaceans now under our consideration. They are often found, together with the Tanaïs, on the bodies of cetaceans and chelonians, on plagiostomous fishes, or in the midst of colonies of Sertulariæ. They also establish themselves on buoys when they are well covered with animal life; and we have discovered them in prodigious numbers on a piece of cable which had lain at the bottom of the sea, and the whole surface of which was covered with animals of every kind.
We may here mention the Pycnogonons, the Saphyrinæ, the Peltidiæ, and the Hersiliæ; these crustaceans often crawl over the skins of their congeners, but without ever renouncing their independence; and they are all more or less occupied with the toilet of their neighbours.