In some of the lower animals the nerves terminate on reaching the skin at the base of rod-like structures similar, in many respects, to the rods of the retina, or the auditory rods of the ear, and of which it is very difficult to say whether they are organs of touch or of some higher sense.
Round the margin of the common sea-anemone is a circle of bright blue spots, or small bladders. If a section be made, there
Fig. 2.—Ceratius bispinosus ("Challenger Reports," vol. xxvii).
will be found a number of cylindrical organs, each containing a fine thread, and terminating in a "cnidocil"; and, secondly, fibers very like nerve-threads, swelling from time to time with ganglionic expansions, and also terminating in a cnidocil. These structures, in all probability, serve as an organ of sense, but what impressions they convey it is impossible to say.
Some jelly-fishes (Trachynemadæ) have groups of long hairs arranged in pairs at the base of the tentacles (Fig. 3), which have been regarded as organs of touch, and it is certainly difficult to suggest any other function for them. They are obviously sense-hairs, but I see no reason for attributing them to the sense of touch.
The so-called eyes of the leech, in Leydig's[1] opinion, which is confirmed by Ranke,[2] are also developed from the supposed special organs of touch. The latter are much more numerous, as many as sixty being developed on the head alone. They are cylindrical organs, lined with large nucleated refractive cells, which occupy nearly all the interior. A special nerve penetrates each, and, after passing some way up, appears to terminate in a free end.
I may also allude to the very varied bristles and cirrhi of worms, with their great diversity of forms.
