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Another beautiful trematode, the Octocotyle lanceolata, lives abundantly on the gills of the alosa, and another, the Octobothrium merlangus, on those of the whiting. The gills of the Mustelus vulgaris regularly bear another species resembling a leech, but instead of a single sucker there are six; this is the Onchocotyle appendiculata.

The bladder of frogs lodges a very beautiful and large trematode which has lately been studied by many naturalists, the Polystomum integerrimum. Many observations remain to be made on the different phases of the existence of this parasite. Its organization is known, and it has been seen to lay large and beautiful eggs, but its movements have not been observed before its entrance into the bladder.

This Polystomum of the frog-and it is no doubt the same with the species Polystomum ocellatum which inhabits the mouth of the European tortoise (Emys Europea)-lays eggs only in winter, and the eggs of the young ones do not seem to produce more precocious embryos than those of the adult. The embryos are ciliated, unlike those of many of the ectoparasite worms. They much resemble the gyrodactyles, especially by their bristles; and like these, they inhabit the cavity of the mouth before they migrate into another organ. We may even ask if these singular gyrodactyles, so peculiar in many respects, are not the larval forms of trematodes allied to the polystomum.

Several important works have lately appeared on the Polystomum integerrimum, by Mons. Stiéda in 1870, by Mons. E. Zeller and Mons. Willemoes-Suhm in 1872.

The gyrodactyles, which we have just mentioned, are

among the most curious covered during late years. live in the gills of fishes, often in great numbers, and move with considerable agility. They are armed with very variable hooks, which serve to anchor them; and sometimes a digestive canal and organs of sensation are found in them.

worms that have been disThey are of small size, and

The Gyrodactylus elegans bears within it a young one which already has hooks, and in this young one, which is not yet born, we see another generation with the same organs, so that three generations are thus enclosed. The daughter is ready at the moment of her birth to give birth to another daughter. According to another mode of interpretation, the mother and daughter are sisters; the elder is found at the periphery, the younger at the centre. These worms are found abundantly in the gills of the cyprinidæ, or white fishes. We have only to scrape gently the surface of the gills with a scalpel, and thus remove a small quantity of a mucous substance, place it on a slide of a microscope, cover it with thin glass, and examine it immediately with the compound microscope. We cannot repeat this three times without finding gyrodactyles.

There are also many insects which live as parasites on plants, and demand from them both a resting-place and their food. Almost all the Hemiptera are among these; we have already mentioned them. The hemiptera, which live on the sap of vegetables, are parasites in the same manner as those which live at the expense of animals. We ought not to make a difference between. the manner of life of the bugs of plants and those of animals. It may be said that Providence has placed

these beings as riders on both the vegetable and animal kingdoms to restrain them with a bridle. What the

gardener does to plants, the aphis has often done before in order to arrest a too vigorous and rapid growth.

The cochineal insect (Coccus cacti) Figs. 80 and 81,

Fig. 80.-Cochineal insect, male (Coccus cacti), natural size and magnified.

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originally from Mexico, lives on the cactus nopal as a true parasite, and furnishes a precious colouring matter, carmine. This insect has been introduced into

I

Fig. 81.-Cochineal insect, female.

Fig. 82.-Aphis.

the Antilles, Spain, the Canary Isles, Algeria, and Java.

Lake is produced by a species of the same genus, originally a native of India (Coccus lacca).

Aphides (Fig. 82) feed on the sap of plants; they multiply rapidly without the male insect. Rose-trees, and more especially their buds, are attacked by a species of a green colour, of which we give a representation (Fig. 83).

An aphis, the Phylloxera vastatrix, has, a short time since, invaded the vineyards, and small as it is, it is dreaded as a plague which scatters ruin in its path. According to recent observations this insect has a double series of generations which precede each other: the mother type and the tubercular type. But this polymorphism seems to be more apparent than real,

although there is a considerable difference in their manner of life and of procuring nourishment. Is this difference the result of the different kinds of food

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thing which may reassure us as to the future attacks of

Fig. 83.-Rose-Aphis. Male and female.

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