The bat of the high mountains of Bavaria, known under the name of _Vespertilio mystacinus_, harbours a nematode, the _Rictularia plagiostoma_, the same which is found in Egypt in the hedgehog (_Erinaceus auritus_). The bat on the banks of the Rhine has not this remarkable worm. We must therefore conclude that the bat of Bavaria finds and eats the same insect as the hedgehog in Egypt, and that this insect does not live on the banks of the Rhine. We have never met with this nematode in the mystacines of Belgium, and yet we have opened them by hundreds. A bird found in Florida, the Anhinga, has in its brain a nematode whose presence in that organ is not accidental. The _Echinorhynchi_ form a very remarkable group of parasites. They migrate from one host to another; but the vehicle by which the greater part of them is conveyed is not known. We represent in Fig. 72 a species which is very common in the intestine of the sprat. It is known that these worms migrate when young, and undergo metamorphoses when they change their host. The _Asellus aquaticus_ of fresh water, harbours besides other worms, the _Echinorhynchus hoeruca_; the _Gammarus pulex_, another fresh-water crustacean, lodges the larva of the _Echinorhynchus proteus_ (Fig. 72). We commonly find this beautiful species of the Echinorhynchus in the alimentary cavity 252 of the sprat, and it is easily distinguished by its peculiar form and its orange colour. [Illustration: Fig. 72.--_Echinorhynchus proteus_ of the Sprat.] [Illustration: Fig. 73.--Sac with psorospermiæ, in the _Sepia officinalis_.] The _Asellus aquaticus_ seems also to serve as the vehicle of the _Echinorhynchus angustatus_. The hooks of the embryos differ from those of the adults, as the six hooks of the cestodes differ from the crown of the adults. Leuckart has described those of the envelope of the _Echinorhynchus proteus_ and the _Echinorhynchus angustatus_. The embryo of the Echinorhynchus has only two large hooks on each side, but several smaller ones. The two species mentioned above have on each side five or six hooks placed at right angles with the median line, but they are not all of the same size. The animals are allied to the _Gordii_ in their development. In fact, their development is like that of the echinodermata; the larva is the _Pluteus_, in which the true echinorhynchus develops itself, borrowing the skin of the pluteus. According to the experiments made by Schneider, the larvæ of cockchafers must be the vehicles of the 253 _Echinorhynchus gigas_. Pigs disseminate the eggs, and the embryos infest these larvæ, in the bodies of which they pass through their principal changes.