-dz

interior of which lies coiled up a long filament, which is often serrated at the end. Even a very slight pressure causes this thread to spring out, and these little darts, which are present in immense numbers in the skin of Hydrozoa (jelly-fish, etc.), serve both as weapons of defence and also to wound the small animals on which they feed.

nz represents a nerve-cell, and it will be seen that the hair in which it terminates does not materially differ from the rest.

In the Annelides, also, the general surface of the integument (Fig. 15)

-nz

Fig. 14.-Diagram of part of the skin of a sea-anemone (Actinia); after Korotneff. dz, Glandular cell; nz, nervous cell.

presents tactile setæ or ciliæ, which are scattered over

Fig. 15.-Anterior part of body of Bohemilla comata (after Vejdovsky *). lb, Tactile hair; hp, hypoderm; c, cuticle; b, anterior part of brain; a, eye; ne, nervefibri.s; v, anterior blood-vessel.

the surface, and especially on the head. In some cases

* "Syst. und Morph. der Oligochoten." 1884.

these setæ are collected into special groups, either situated in cup-shaped depressions of the skin, or on more or less elevated papillæ. Fig. 15 represents the anterior part of the body of a small fresh-water worm (Bohemilla), and shows clearly the small cuticular, and the larger tactile, hairs. In other cases, as in the feelers and cirri of the Alciopidæ, there are short, shining, ovoid rods, to the base of which runs a nervous fibril.

In the Mollusca, also, the surface of the skin is very sensitive, and is generally provided with minute setæ, especially on the tentacles, or as in Lamellibranchiata (mussels, etc.), on the edge of the mantle. In some, the snail for instance (Helix), the nerves, on approaching the skin, have been ascertained to divide into a plexus of fibrils.

Fig. 16.-Diagrammatic section through a papilla of touch of Onchidium (after Semper). a', a", Two layers of the cuticle; a, biconvex thickened portion of the cuticle; b, enlarged epithelial cells; b', ordinary epithelial cells; c, cellular body; d, cells; n, nerve.

In Onchidium, a genus of slugs, Semper describes as organs of touch (Fig. 16) certain slight elevations of the

skin caused by the cuticle being somewhat thickened. Beneath these the epithelial cells are larger than usual; and under them, again, lies a cellular mass, the minute. structure of which he was not able to determine, but which is connected with a nerve.

On the mantle of the Chitons are also certain well-defined organs, probably of touch. They occupy pores in the shells, and resemble obconical or somewhat dice-box shaped plugs of transparent, highly refracting

Fig. 17.-Diagram of the structure of the soft and some of the hard parts in the tegmentum of a shell of a Chiton (Acanthopleura spiniger), as seen in a section vertical to the surface and with, the margin of the shell bordering on the girdle lying in the direction of the left side of the drawing. f, Calcareous cornea; h, iris; g, lens; k, pigmented capsule of eye; n, optic nerve; r, rods of retina; n', branches of the optic nerve, perforating the capsule wall, and terminating in b', b', b', ocular sense-organs; p, p, nerves to sense-organ; m, body of sense-organ cut across; a. p, fusiform body of sense-organ entire; a, obconical termination of senseorgan; e, nerve given off by one sense-organ to another, b'.

tissue. The terminal knobs end in flat discs, which show a series of concentric rings, as if composed of a series of concentric layers or inverted cones fitted one within the other.* Each one terminates in a nerve

*

16

CRUSTACEA AND INSECTS.

fibre. They are of two distinct sizes, which Moseley proposes to call macroesthetes and microsthetes.

In many animals, as in ourselves, the outer skin is soft and susceptible to external impressions. In Insects and Crustacea on the contrary, the inner skin, or hypoderm, is covered with a more or less thick layer of horny substance known as chitine; and, from the nature of their chitinous integument, it naturally follows that the sensations of insects, excepting that of sight, are effected by means of variously modified hairs. We know, however, so little, in the first place, as to the real means by which animals, including man, hear, smell, or taste, and, in the second, as to the intimate structure of their minute organs, that we are often in doubt, and there are still great differences of opinion whether a given sense-hair serves for hearing, smell, or touch.

The hairs of Arthropods belong to very different

Fig. 18.-Diagram of forms of hairs in insects. a, Ordinary surface hair; b, plumose natatory hair; c, hair of touch; d, auditory hair; e, olfactory hair; f, taste hair; n, nerve hair.

categories, some of which we may perhaps distinguish as follows:

Those under which the chitinous integument is entire. 1. Ordinary surface hairs (Fig. 18, a).

2. Plumose natatory hairs (Fig. 18, b).

Those under which the chitinous integument is

per

forated, and a special nerve-fibre runs to the base of the hair.

1. Hairs solid.

(1) Hairs attached stiffly; organs of touch (Fig. 18, c).

(2) Hairs attached by means of a thin membrane, sometimes plumose; organs of hearing (Fig. 18, d).

2. Hairs hollow, and either open at the end, or closed by an extremely delicate membrane.

(1) Hairs containing a

nervous plasma;

organs of smell

(Fig. 18, e).

(2) Hairs generally

very short, and

continuation of the

Fig. 19.-Part of the proboscis of a fly (Musca); after Leydig. n, Nerve; g, ganglionic swellings; s, tactile hairs or rods; c, cuticle.

the tactile hairs on the proboscis of a fly (Musca), each seated on a ganglion and connected with a nerve (n).

C