That the otocysts are concerned with equilibrium and adjustment of movement has been definitely proved by experiments upon the Prawn (Palamon). When this creature moults it sheds not only the defensive armour of the body but also the lining of A B Fig. 1045.-Otocyst of Lobster (A) in longitudinal section, enlarged and diagrammatic; sensory bristles are seen projecting into its cavity, which contains numerous oto the otocysts, getting rid at the same time of the sand grains which serve as otoliths. Under ordinary circumstances these would be replaced by a fresh supply of the same material, but the specimens experimented upon were only provided with iron filings, some of which in due course were introduced into the otocysts. It was then found possible by means of a magnet to move the particles in various ways, and as a result of this the Prawns could be induced to assume all sorts of positions, under the impression, so to speak, that they were falling over in this or that direction, which they would have been if the shifting of the otoliths had been produced by ordinary causes. liths; Nv., nerve. B, A sensory bristle, further enlarged Fig. 1046.-Opossum Shrimp (Mysis), cnlarged. One of the otocysts (EAR) is seen in the tail In one Crustacean, the Opossum Shrimp (Mysis), the otocysts, in this case closed, are lodged in the flaps of the tail-fin, but why they should have this position is not known (fig. 1046). It is generally assumed that animals which are endowed with a voice or its equivalent also possess powers of hearing, at least if the voice is used for the benefit of one another. Since some nerve lig. ch.n Im ch.l. ch. gn. of the higher Crustaceans are able to emit sounds, it is quite possible that their otocysts are beginning to acquire a new use, i.e. that of serving as auditory organs. The Rock-Lobster (Palinurus), for example, makes a creaking noise by moving the basal joints of the large feelers, which then rub against their sockets. An unpleasant sound of similar nature can be produced by twisting a glass stopper in the neck of its bottle. A more specialized case is that of the Musical Strand-Crab (Ocypoda macrocera), which has been described by Alcock (in A Naturalist in Indian Seas). In this animal the inner side of the large nippers is provided with a ridge or scraper placed near the base of the limb, and a rasp-like ridge or key-board on the fixed joint of the claw. By drawing the scraper over the keyboard a sort of chirping sound is produced, not unlike the one with which our native grasshoppers have made us familiar. The same zoologist speaks On the left is seen part of the chordotonal organ with its rods, ganof the Squeaker Crab (Psopheticus stridulans) of the Andaman Sea as making a dismal noise by rubbing a spine which projects from the base of its nippers against a rough knob near the eye-socket. rods! 1.b. ch. Fig. 1047.-Chordotonal Organs. On the right is shown part of an abdominal segment of the larva of a Gnat (Corethra plumicornis), seen as a transparent object, enlarged. In the centre is the nerve-cord (darkly shaded) with the ganglion (gn.) of the segment; l.m., longitudinal muscles; ch.n., ch.g., ch.l., and ch., chordotonal nerve, ganglion, ligament, and organ; t.b., branched tactile bristles. glion (gn.), nerve, and ligament (lig.) still further enlarged. ORGANS OF BALANCE AND HEARING IN INSECTS (INSECTA).— A variety of organs situated in different parts of the body are probably connected with balance or hearing, or both. Among those which are most likely concerned with equilibrium and movement are certain peculiar structures (chordotonal organs) that are especially characteristic of aquatic larvæ, though not limited to these. Gnat larvæ, for example, possess such organs, one of which is represented in fig. 1047. It consists essentially of a group of rod-shaped cells contained in a tube that opens to the exterior. Many insects make sounds which are doubtless heard by their fellows, a well-known instance being afforded by Grasshoppers and Crickets. A Grasshopper possesses a chirping A B с D 10-4 Ear 32 1 arrangement something like that of the Musical Strand-Crab (p. 37). The wingcover is provided with a sharp edge or scraper which is rubbed along a key-board placed on the inner edge of the thigh of the hind-leg (fig. 1048). The chirping 66 sounds audible to our own ears are produced by the male insect, but the females of some species are also provided with these stridulating" organs, which no doubt make sounds that can be heard and appreciated by the opposite sex. These sound-producing insects also possess what we may feel justified in calling "ears". On either side of the first ring of the abdomen there is a membrane comparable to a drum-head (fig. 1048) stretched over an air-space, and closely connected with sensory arrangements somewhat like those already described for a gnat-larva. The ears of Green Grasshoppers and Crickets are situated in the shins of the fore-legs, just below the knee. Fig. 1048.-Stridulating Organs and Ears of Grasshoppers. A, Hind-thigh of a male Grasshopper (Stenobothrus), showing beaded key-board (dotted line on right), enlarged; B, five beads of same, further enlarged; c, six of the much smaller beads of key-board of a female (same scale as B); D, side view of a Grasshopper (Acridium), to show ear; wings and legs cut short; 1-3, thoracic stigmata; 4-10, abdominal stigmata. n. ORGANS OF BALANCE AND HEARING IN BACKBONED ANIMALS n. C. g. Fig. 1049.-Body of an Ascidian Tadpole, in longitudinal bably a balancing organ (fig. section, enlarged. The tail is cut short; the dorsal nerve-tube (n.t.) swells into a brain, into which project a balancing organ (b.) and an eye (e.): a., atrial cavity: c., groups of embryonic cells; g., gut; m., mouth; ., notochord; p., adhesive papillæ. broadly speaking, to the essential 1049). From Fishes onwards we find undoubted ears, similar, parts of our own organs of hearing, and there can be no doubt that these also have to do with equilibrium and movement. If we trace the development of the น. S. S. p.8. S. S. h. S. C. 0. ear we shall find that it begins as a pit in the skin, and by closure of the mouth of this a vesicle is produced, which if it underwent no further modification would be called an otocyst. As it is, however, a very complex shape is assumed, the final result being known as the membranous labyrinth, or internal ear (fig. 1050). This sometimes, as in a Skate or Dog-Fish, remains in communication with the exterior throughout life. It is significant that in Fishes the auditory pit arises in close connection with the "lateral line ", which is a groove or tube containing groups of sense-cells belonging to the skin. And this suggests that the ear is no more than a bit of this line which has sunk beneath the surface and become specialized as regards structure and function. It is extremely probable that the lateral line of Fishes and Amphibian larvæ has to do with maintenance of balance and direction of movements, and if so, the fact that the ear has to do with these functions is quite intelligible. We know so little about the division of physiological labour between the different parts of the complex labyrinth that a discussion of details would be out of place here. But experiments have shown that the semicircular canals S., S. (and other dotted patches), groups have some connection with movement and equilibrium, and it is interesting to note that they lie in three planes which are mutually at right angles. It is also certain that the labyrinth is the sense-organ of hearing proper. In land-vertebrates there are more or less perfect arrangements for conducting air-waves from the exterior to the deeply-seated and well-protected internal ear. This has already been sufficiently illustrated by the brief account of the auditory organs of Man given elsewhere (see vol. i, p. 56). 46 SIGHT 0. sa. Fig. 1050.-Diagram of the Left Membranous Labyrinth of a Lower Vertebrate, seen from the outer side. t., A tube re presenting the outer part of the original tical, posterior vertical, and horizontal ingrowth; a.s., p.s., and h., anterior ver semicircular canals; u, utricle; sa., saccule; c., cochlea; o., 0., otoliths; s., s., s., of sense-cells. SKIN-SEEING. The simplest kind of sight is literally that of seeing without eyes", and it amounts to no more than the power of distinguishing between light and darkness, or detecting sudden variations in the amount of illumination. But even this limited sort of vision may be of the greatest importance to its possessor, since it often gives valuable information about the surroundings. In such skin-seeing (dermatoptic vision) it is usual to find colouring-matter or pigment in or below the epidermis, which localizes the action of light-rays upon sensitive cells in this layer. This is the case, for instance, in Earth-Worms, the safety of which must often depend upon their avoidance of light. A further and more interesting illustration is afforded by many of the bivalve Molluscs which live in sand or mud, and which feed and breathe by means of two tubes, the siphons, which project from the hinder end of the body (vol. ii, p. 249). Such animals are Eye A m. -N. -p.v B often found hidden in their burrows with only the extreme tips of the siphons projecting. But even though thus concealed they would more frequently fall victims than they do to octopi and fishes, or, in the case of those which live between tide-marks, to strand-haunting birds, were they not provided with some means of detecting the proximity of such enemies. Warning is often given by the siphons themselves, which are commonly pigmented and sensitive to changes in light-intensity. And experiments on specimens kept in aquaria have shown that the fully-extended siphons are rapidly drawn in if a shadow is suddenly cast upon them, an event that would happen under natural conditions on the approach of a voracious fish or too inquisitive bird. Fig. 1051.-A, Euglena viridis, enlarged; fl., flagellum; m., mouth; N., nucleus; p.v., pulsating vacuole. B, Eye-spot, greatly enlarged, showing six rounded lenses resting on a mass of pigment. EYES.-Localization and improvement of the powers of sight have led to the evolution of definite visual organs or eyes, though many of the lower Invertebrates have more or less retained the old faculty of diffuse skin-seeing. The simplest organs of the kind are known as eye-spots, and their presence is marked by dense pigment. These are possessed even by some Animalcules, e.g. by a little green creature (Euglena viridis) which often swarms in stagnant water (fig. 1051). The eye-spot in this case is marked by the presence of a tiny patch of red colouring-matter on which rest several little lenses that serve to concentrate the light. In some of the Jelly-Fish the margin of the umbrella bears |