Zoologists take a special interest in the crayfish because it is found to be a most convenient type for the purpose of teaching the principles of zoology to young students, and with that end in view was made the subject of a very beautiful little book by the great teacher Huxley. The conclusions above stated in regard to the gills are set forth in that book with admirable illustrative drawings, and the striking fact of the dwindling and suppression of the various gill-plumes is clearly explained. [Illustration: FIG. 33.–The rudimentary gill-plume of a crayfish from that part of the body-wall to which the first pair of jaw-legs (maxillipedes) is articulated. Found in the red-footed crayfish (Astacus fluviatilis) but in no other species of Astacus. It is one-fifteenth of an inch long. Drawn by Miss Margery Moseley in 1904. ("Quart. Journal of Microscopical Science," vol. 26 (1904-5).)] And now we come to an interesting discovery in this matter of the gill-plumes of crayfishes. Some fifteen years ago the daughter of my friend and colleague–Professor Moseley–was a member of the class of Elementary Biology at Oxford. She had to examine and identify these and other points in the structure of the crayfish. The class was supplied with specimens of the French red-footed crayfish "Astacus fluviatilis," as it is more readily obtained from fishmongers than our own "pale-foot" or "Astacus pallipes." She found in her specimen far forward on each side of the "head" a very minute gill far away from the others and previously unknown. The demonstrator in charge of the class refused even to look at her discovery. So she confirmed it by examining three other specimens–made drawings of the tiny branched gill (as shown in Fig. 33) and their position, and sent them to me in London. It was at once clear that she had discovered in this much studied little animal a very interesting pair of gills (right and left)–unknown to Huxley and the rest of the zoological world. She proceeded to examine specimens of A. fluviatilis from various rivers of Germany and France and always found the new gill-plume. She also showed (I supplied her with specimens at the Natural History Museum) that it was, on the other hand, absent from A. leptodactylus, A. pallipes, and all the foreign species (some from Asia) which are known, and she published an illustrated account of it in the "Quarterly Journal of Microscopical Science." This tiny gill-plume is placed very far forward on each side of the body, the farthest point forward at which any gill-plume is found in any kind of prawn, shrimp or lobster, namely in the region where the first pair of jaw-legs is attached, so that there are three empty spaces between it and the rudimentary gill over the fifth pair of legs, already known in the red-footed crayfish. It is only two millimetres long–about one-fifteenth of an inch! But its presence serves very distinctly to separate the red-footed crayfish,