life; and it is not a matter of doubt to us that parasites often play their allotted part in the economy; their absence as well as their presence may be the cause of inconvenience. We should not even be astonished if the administration of certain worms internally should be prescribed as a remedy. Have we not known the time when all maladies were supposed to yield to the action of leeches, and do we not see the good effects of their application? There are many kinds of parasites, and their therapeutic effect may, perhaps, in future, form an interesting subject of study.
To speak at the present time of a verminous temperament would be scientific heresy, an anachronism; this shows the progress that we have made of late years. Valenciennes was permitted to employ this language at the Academy of Sciences in Paris not twenty years ago, and Lamarck wrote thus in his standard work on invertebrate animals, in the beginning of this century: “It is very certain that there exist in a great many animals, and even in man, intestinal worms; some of which are formed there, others are born and all live there, multiplying more or less, without any of these worms showing themselves externally, or being able to live elsewhere.
“During so many centuries that observations have been made, well-ascertained species of intestinal worms have been found nowhere else than in the bodies of animals. We are now authorized to believe that there are innate worms, or such as are produced by spontaneous generation, and that these are modified from time to time; this is at present the opinion of the most enlightened observers.”
Thus it was considered by Lamarck that parasitical worms are only found in the bodies of animals, and are actually produced there.
Can it be believed that such ideas were put forward by zoologists of the highest merit? and ought we to feel surprised that the theory of spontaneous generation was so long taught in the physiological schools?
A book published in 1859 was entitled, “Heterogenesis, or a Treatise on Spontaneous Generation.” The author gives the clue to the origin of his errors in the second line of his preface, in which he says: “When, by meditation, it was evident to me that spontaneous generation was one of the means employed by matter for the reproduction of living beings.”... According to this philosopher, science is, therefore, not the generalization of facts, but these facts must serve to prop up the theories or hypotheses invented in the silence of the study. This passage of his work shows us that he was no more able to yield to the evidence of experiments made on worms, than to those of Pasteur on the infusoria.
It may be related to the honour of the illustrious Baer, that, from the year 1817, during his stay at Königsberg, he took up arms against this hypothesis, and never ceased to combat it, till evidence succeeded in opening the eyes of the most obstinate.
The worms which present the most remarkable phenomena of transformations, accompanied by metamorphoses, are the Distomians and Cestodes, flat worms, which we will consider in the first place.
Trematode worms include a certain number of large and beautiful parasites which scarcely undergo any change, and are found only on the skin and the gills of
certain fishes; these are the monogenetic trematodes, comprising the Tristomidæ and all the worms of that group, which also stand higher in their organization: we shall speak of them hereafter. The other trematodes, which are called digenetic, live on the most dissimilar animals, under the most varied forms, and, like the greater part of the cestodes, introduce themselves into the individual who is to give them shelter, only by the assistance of a host, acting as a stage-coach which serves them as a vehicle.