THE LOCATION OF THE SENSE OF SMELL The sense of smell is, usually, only very indefinitely located in invertebrates. In these animals, the sensory cells which react to chemical stimuli are clearly dis tributed over large areas of the skin. This applies particularly to Coelenterates, the more lowly of the worms, and the Echinoderms. In the last-mentioned animals, particularly in starfishes and ophiuroids, the chemical sense can be proved to be present in the thousands of ambulacral feet. Some ophiuroids bury themselves in the sand,, leaving only the ex tremities of the arms projecting. The animals scent meat placed near them by means of the am bulacral feet which are situated in these parts.
Even animals as highly organ ised as cuttlefishes show a wholly diffuse distribution of their chem ical sense. (Giersberg.) It can be traced right into the extremities of the arms, the suckers being the most sensitive parts. On the other hand, the sensory pits situated behind the eyes, which, in former works, were described, without hesitation, as olfactory organs, have nothing to do with the chemical sense.
The gill-breathing gastropods have, near the gills, an "osphradium," an oval thick ened patch of epithelium, having many transverse folds (fig. 3). In some gastro pods it has been proved experimentally that the osphradium is used in searching out food (Copeland). The animals used for the experiments, Allectrion obsoleta and Busycon canaliculatus, when crawl ing, wave their long respiratory siphon, by means of which they inhale water, rapidly to and fro. If, in this process, they inhale water which has an odour agreeable to them, they immediately swerve from their path, and are able to find the piece of food whence it emanates. This behaviour does not take place if the osphradium is removed by an operation. Nassa reticulata, which is common in European seas, buries itself almost com pletely in the sand in daytime, leaving only its long siphon projecting. If pieces of some bivalve are placed in the aquar ium, in a very short time all the Nassae leave their retreats, and search for the food.
In insects, the feelers are the olfactory organs. Forel had proved already that, in these animals, the sense of scent is completely wanting after loss of the feelers. Later, v. Frisch in particular has occupied himself with this problem. He found that the olfac tory organs of the bee are situated in the last eight joints of the feelers. Seven of these joints can be cut off without the bees' perception of odours being greatly damaged. When the eighth is cut off, however, all sense of smell is lost. In Dytiscus the olfac tory sense is located in the end joint of the maxillary palp, as well as in the feelers.
The microscopic structure of the olfactory organs in the feelers of insects is shown in fig. 5. The most important parts are the pore-plates, besides which the olfactory pegs and the pit-pegs are to be distinguished. The chitin is extraordinarily thin at the point where the sense organ is situated. A fine bundle of nerve fibres connects the organ immediately with the surface. The knowledge that these particular structures are the olfactory organs is not, by any means, drawn solely from their form.
Von Frisch was able to prove definitely in the bee that the pore-plates are present only on those joints, the amputation of which is followed by the disap pearance of the olfactory sense.
In the higher Crustacea the chemo-receptors are situated on the outer ramus of the ist anten na, on the mouth parts, and, in deed, also in the mouth cavity. On the antennae they take the form of olfactory hairs or olfactory pegs. These are very fine cylindrical structures, which are traversed, just as are the olfactory pegs of insects, by a bundle of the finest fibres, which appear to be extensions of the more deeply seated sensory cells.
In Crangon the 1st antennae occupy a particularly favourable position. When the animal is buried in the sand, they alone pro ject from it, and thus to them falls the task of scenting possible prey. For this reason normal ani mals react to meat juice very much quicker than those which have no antennae. On the other hand, it can be shown that the antennae are not more sensitive than the more concealed sensory cells of the mouth parts, indeed, in certain respects, the antennae appear to be of even less im portance. Stimulation of the an tennae only never causes the typical food reaction, in which the crustaceans leave their hiding places, and seek the meat ; to cause this, it is necessary for the mouth parts also to be stimulated.
The olfactory organ of the blood-sucking ticks (Ixodes) is in a particularly remarkable position. It is situated on the tibiae of the fore-legs, and is usually called "Hal ler's organ." Ticks, however, use their fore-legs, not for running, but as feelers; they elevate them and stretch them out in front of them. The olfactory organ, natur ally, primarily aids the insect in finding its host. Such a faculty is particularly neces sary to such creatures as the pigeon ticks (Argas reflexus) which, by day, hide away in some part of the dove-cote, and attack their hosts only by night. If the front legs are amputated, they lose all sensitive ness to chemical stimuli. (Hindle and Merriman.) They will then suck their fill of any warm liquid which may be offered to them in a suitable manner.