After what has been said of the Prawn, little space need be devoted to the Shrimp, for it may be gone over in precisely the same way. It will be sufficient to call attention to the difference in the antennae, to the rudimentary rostrum or beak, and to emphasize the distinction between the terminal joints of the first leg in the two creatures. The leg shown in Fig. 67 corresponds to the limb used for cleansing by the Prawn. There is a great difference in their habits, for Shrimps burrow in the sand for concealment. In doing this the swimming feet, as well as the walking legs, are brought into action, and when the Shrimp is settling down, sand is swept over its back by the antennae, to render the concealment complete. In many of the rock-pools round the coast, and also in brackish water, Mysis, or the Opossum Shrimp, may be met with. It is not, so far as my experience goes, a good inmate of the aquarium, but it is extremely interesting from the fact that, unlike its higher relations, the auditory apparatus is not situated in the antennae, but in the plates of the telson (Fig. 68E). [Illustration: FIG. 68.--Mysis, or the Opossum Shrimp.] Mysis is shrimp-like in general appearance but differs from Shrimps in the structure of the legs, in the absence of gills, and in other particulars. The telson consists of five pieces. In each of the two inner and smaller pieces is an oval sac, like that described in the basal joints of the first antennae of the Prawn, containing a single lens-shaped otolith, consisting of chalky matter embedded in some organic substance. ‘The vibration of the hairs [in this sac] is mechanical, not depending on the life of the animal. Hensen took a Mysis, and fixed it in such a position that he could watch particular hairs with a microscope. He then sounded a scale; to most of the notes the hairs remained entirely passive, but to some one it responded so violently and vibrated so rapidly as to become invisible. When the note ceased the hair became quiet; as soon as it was re-sounded, the hair at once began to vibrate again. Other hairs, in the same way, responded to other notes. The relation of the hairs to particular notes is probably determined by various conditions; for instance, by the length, thickness, &c.[47]’ We shall not be able at present to repeat Hensen’s experiment, but we may break up the sac and extract the otolith, which may be seen with the lenses at our command. Small specimens of the Shore Crab (_Car´cinus mae´nas_) are fair game for us. They will interest us while living in the aquarium, and when dead we can put them into pickle, and break them up at our leisure.