phases of its evolution. It was at first thought that we had before us an echinoderm in the act of transformation. I wrote to J. Müller immediately after the discovery which he hastened to announce to me, to state that in my opinion, this was only a new instance of parasiticism; parasites are, however, so rare in this class of animals, and their mode of life is so exceptional, that one ought not to be surprised that this fact did not receive at first its true interpretation.

Professor Semper found at the Philippine Islands, in the Holothuria edulis, another species of Entoconcha which appears to attach itself to the anal vent of this echinoderm. He gave it the name of Entoconcha Mulleri. We have in it a new example of the relations which certain parasites bear to their hosts, and which are the same in both hemispheres.

The Lichnophora are infusoria, allied to the Vorticellæ, whose form they assume; these are "mimic species," or mocking forms, of the Trichodinæ. One species, the Lichnophora Auerbachii lives on the Planaria tuberculata; the other, the L. Cohnii, on the branchial membranes of the Psyrmobranchus protensus.

The associations in the inferior ranks of animals have functions which are of the highest importance; some to maintain harmony and health in all that possess life, others to sow the seeds of death throughout whole regions. There are, in fact, associations in the ranks of the infinitely small creatures, which sometimes have the effect of purifying and rendering more healthful, sometimes of destroying. It is among these beings, invisible to the naked eye, that we must seek for the cause of some epidemic diseases. We have here an example of

what certain groups of animals are able to accomplish. The crustaceans everywhere perform the office of vultures to clear the waters from dead bodies, whether large or small, and they are in general sufficiently numerous to perform this police duty effectually. We may say that without their aid the waters along the coasts and at the mouth of rivers would grow speedily corrupt and unfit to support life. Thus it sometimes happens that when the number of these beings is insufficient, or the putrescible matter is in excess, we see the fish, the molluscs, and even the crustaceans, perish one after the other.

The last of the parasites of this category are known by the name of Gregarinæ. It appears that Gode was the first to make observations upon them. Léon Dufour gave them the name which they still bear. They have a

very simple organization, and are formed only of a cell which contains a nucleus: they live in the intestines of many invertebrate animals, especially in the articulata. Let us imagine a body, long, more or less transparent, with a smooth surface very like a spindle, which glides about in the intestines, in the midst of the liquid matter which it contains, without our being able to ascertain

the mechanism by which it moves (Fig. 33.) While young they are encysted, and bear the name of Psorospermiæ. Fig. 34 represents one of these sacs of Psorospermiæ from a cephalopod.

The gregarinæ live in their perfect form chiefly in insects, crustaceans, and worms. Fig. 35 represents a gregarina very common in the libellulæ. The largest species inhabits the intestines of the lobster. My son has studied them very carefully, and published the results in the bulletins of the Academy of Belgium.

Schneider has described a parasite which ought, no doubt, to be placed among the gregarinæ; it lives in the testicle, as well as in the salivary cells, of a planaria, the Mesostomum Ehrenbergii; Schneider represents the various phases of its de- Fig. 35.-Stylorynchus velopment. In the autumn of 1871, nearly all the mesostomes. perished through the presence of these parasitical organisms: in the following year they

were rare.

oligacanthus from the larva of the Agrion.

Some years ago, Kölliker discovered on the spongy. bodies of molluscs, certain parasites, the nature of which appears still as enigmatical as on the first day of their discovery. The Würzburg professor gave them the name of Dicyema. We have had for a long time in our portfolio some observations upon them, and at the close of the chapter "On Parasites that undergo Transformations," we give a representation of a Dicyema which we found in abundance on the Sepia officinalis off the coast of Belgium.

M

CHAPTER VIII.

PARASITES THAT ARE FREE WHEN OLD.

We are about to study in this chapter animals which seek for assistance from others while young, and are able to provide for themselves completely when they have grown old. We may compare the hosts which afford

them shelter to crèches which receive none except newborn infants. It is generally supposed that animals known under the name of parasites are such as require assistance from their neighbours during all the stages of their existence.* This is a mistake. There are very few among them which are not able to provide for themselves during some period of their development, and they then lead an independent life. We have mentioned a certain number of them in the preceding chapter, which only seek for external assistance when they are old; we bring together, on the contrary, in this chapter, those which require help at the commencement of their life, and live at large on their own industry when they have once made their entry into the world. There are even some among

* The discovery of a free bothriocephalus at the bottom of a ditch caused a great sensation in the world of naturalists some years ago. It was then thought that the parasite could not exist except in the body of an animal: they could only imagine it shut up as in the cells of a gaol.

them which are richly endowed, and one would never imagine that they would have recourse to strangers in order to bring up their progeny. All their young family

is usually entrusted to the care of a nurse, who lives just long enough to bring them up; she gives them convenient shelter under her roof, and often bestows upon them the last drop of her blood.

When the young one has at last abandoned her first resting-place, she begins to think seriously of Hymen; she changes her dress and her mode of life, and seeks no more extraneous assistance till she lays her eggs. Among the animals brought up in this manner, the most remarkable are the Ichneumons, which have always attracted the notice of entomologists. These charming creatures, whose shape is delicately slender, whose transparent wings flutter with so much grace, have a less stormy youth than their boldness would induce us to suppose. As the cuckoo lays her eggs in the nest of a strange bird, the mother ichneumon deposits hers in a caterpillar full of health, by means of a long and thread-like ovipositor, so that the larvæ as soon as they are hatched, find themselves in a bath of blood and viscera, which serves them for food. The different organs palpitate under the teeth of these intruders, and the young larva grows and increases in size till it is hatched under the skin of its nurse: this skin is the cradle of the ichneumon.

The young ichneumon devours its nurse piecemeal, organ after organ; and for fear that death should supervene too quickly, the mother takes care to chloroform the victim beforehand to make her last longer. The method which many of them adopt to get rid of their