Another Epibdella lives on the skin and on different parts of the body of the European maigre, or the Virgin Mary’s fish; it is covered with pigment spots which cause it still more to resemble the large scales of its host. This fish, which is also called the Sciæna aquila, has its skin covered with similar scales, and they are of the same colour, both on the back and belly.
Another large and fine worm of this group lives on the gills of the sturgeon, and is distinguished by its suckers as well as by its great mobility. The epibdellæ preserve their scale-like form during their greatest contractions, but these worms change with every movement. The Nitschia elegans, for such is the name by which it is distinguished, is not rare on the sturgeon as we see it in our markets. Among the many parasites in this category, there is a very remarkable one which deserves particular mention. It lives abundantly on fresh-water fishes, preferring to attach itself to their gills; it is found most commonly on the bream. For our knowledge of these worms we are indebted to Nordmann.
They bear the name of Diplozoon paradoxum, and are always double, that is to say, always united like Siamese twins, being organically fastened together; they leave the egg, like their congeners, isolated and hermaphrodite, instal themselves separately on their host, and a little time after their choice of a resting-place, they unite so that the tissues, I was about to say the organs, are welded to each other. They cross like two strokes of an x. It is in this position that they live and die, after having produced large and beautiful eggs provided with a very long cable. These eggs are laid separately, and attached to the gills of the fishes which give them shelter. At the end of a fortnight the ciliated embryo comes forth, being provided with two eyes, and seeks to establish itself on a fresh host.
Under the form of Diporpa it has a ventral sucker, and a small papilla on its back, and the two individuals are attached to each other cross-wise by the sucker and the papilla. Notwithstanding what Humboldt says in his “Cosmos,” the Diplozoon is not an animal with two heads and two caudal extremities, but is a double animal, two hermaphrodite individuals united, which at first have lived separately, and have become soldered to each other at the period of maturity.
We find a nematode, and consequently an animal with the sexes separate, which presents the same phenomena. The male and female are soldered together, but the female alone undergoes development. It is the Syngamus trachealis of Siebold. It inhabits the tracheal artery of some gallinaceous fowls, and according to recent experiments, it develops itself directly in the tracheal artery of birds.
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 Europæa)—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