Rathke, a learned professor of Königsberg, was the first to notice an isopod, known under the name of Phryxus paguri, which lives on the stomach of a pagurus, attached to it by its back, so that the stomach of the parasite is turned, like that of the pagurus, towards the partitions of the shell. The tail with the branchial appendages is always directed towards the orifice of the shell. The male is very small and never leaves the female. The Athelca cladophora is another bopyrian living on the abdominal region of a pagurus, which always chooses shells infested by Alcyonia. Another bopyrian, the Prosthetes cannelatus, lives on the abdomen of an ordinary pagurus.
Fig. 29.—Phryxus Rathkei. A figure of the natural size is given at the side.
Mons. Bucholz has recently described a new kind of isopod, allied to the lyriopes, which lives on the Hemioniscus. This isopod fixes itself to a Balanus (B. ovularis), and the female preserves only four of her segments with their appendages: she had fifteen, when young. Thus she throws off nearly all her appendages which have become useless. The male of this isopod, which inhabits the bay of Christiansand, is not yet known. Another parasite of this group has been observed by Fr. Müller at Desterro, on the coast of Brazil. It bears the name of
Entoniscus porcellanæ. The parasite which he discovered by the side of it on the same animal, and to which he has given the name of Lerneoniscus, had perhaps introduced it. We have seen examples of this kind among insects. Among the rich materials which Professor Semper brought back from his voyage, there was a Porcellana, which harbours on its exterior surface a very remarkable isopod, whose recurrent development is no less decided than that of the peltogasters. Dr. Kausmann has lately described these curious organisms, to which he has given the name of Zeuxo. Another isopod, with a no less decided recurrent development, has received from the same naturalist the name of Cahira Lerneodiscoïdes.
We now come to an isopod which aims higher: he doubtless considers that cray-fish and crabs walk too slowly for him; he therefore addresses himself to a fish, the Puntius maculatus, which inhabits the river Tykerang (Bandong) in Java. This isopod is called Ichthyoxenus Jellinghausii. This isopod crustacean, living at first in the same manner as the rest, looks out for a small cyprinoid fish, thrusts itself like a trocar behind the abdominal fins, through the scaly skin, and penetrates entirely into the abdominal cavity. The male always accompanies its female. It is remarkable that she, in contradistinction to many others, preserves all the attributes of her sex. She does not change her form more than the other free crustaceans of her order, and only differs from the male in size. It is well known that in all these animals the male is always smaller than the female. Mons. Jellinghaus, who first described this crustacean, observed that all fishes which he caught had,
without exception, the small ones as well as those which were larger, a couple of these parasites in their stomach. We allude to it here, but we might as well call this Ichthyoxenus a messmate as a parasite.
On the coast of Brittany, among the many Labri, which are distinguished for their vivacity, and for the variety of their colours, is found a small species (Labrus Cornubiensis), on which is usually seen an isopod which is no less curious. It is constantly clinging to the sides of this fish, not far from the head, at the bottom of a hollow made under the scales. Naturalists have known this acolyte by Mons. Hesse’s works.
This Leposphilus (for this is the name which has been given to it), though it does not prefer the scales to any other organ, forms a lodging for itself in the sides of this little Labrus, and takes up its abode there with its family. We cannot assert that it has chosen this refuge without any hope of returning, since both the sexes still keep their organs of locomotion.