higher forms by the fact that the locomotive bristles are comparatively few in number, hence the modern name of the order (Gr. oligos, few; and chaite, a bristle). In the common earthworm (Lumbricus terrestris) the body is cylindrical, attenuated at both ends, and furnished with eight rows of locomotive bristles. The mouth is destitute of teeth, and opens into a gullet which leads to a muscular crop, succeeded by a second muscular dilatation or gizzard. The intestine is continued straight to the anus, and is constricted in its course by numerous transverse partitions springing from the walls of the bodycavity. The pseudo-hæmal system is well developed; and there exists in even greater numbers than in the leeches the series of segmental organs, or lateral pouches, which open externally by pores. The Naïdida are chiefly noticeable on account of the power which many of them possess for producing fresh individuals by a process of budding before they attain sexual maturity. One of the commonest of them is a little worm (Tubifex rivulorum) which occurs abundantly in many of our pools and streams, and which exhibits a fine red colour, owing to the pseudo - hæmal system being visible through the transparent integument.

In

ORDER III. TUBICOLA.—The Annelides included in this group derive their name from the fact that they have the power of protecting themselves by means of tubes (Lat. tuba, a tube; and colo, I inhabit). some cases (fig. 74) the tube is composed of carbonate of lime, and is a genuine secretion from the body. In other cases, the tube is composed of grains of sand or pieces of broken shell, cemented together by a glutinous secretion from the surface of the body. In all the Tubicola the respiratory organs are in the form of branched fila

Fig. 74.-Tubicola. a Serpula contortuplicata, showing the branchia and operculum; b Spirorbis communis.

mentous external gills, in which the fluid of the pseudo-hæmal system is subjected to the action of the outer water. They are

therefore "branchiate" Annelides. As they live in tubes, however, and do not voluntarily expose more than the anterior end of the body, the branchiæ are all placed on or near the head. The filaments of which the gills are composed (fig. 74, a) are richly ciliated, and as the pseudo-hæmal fluid is usually red, they have generally a beautiful scarlet colour.

The most familiar of the Tubicola is the Serpula (fig. 74, a), the contorted and winding tubes of which must be known to every one as occurring on shells or stones on the sea-shore. One of the cephalic filaments in Serpula is much developed, and its extremity forms a kind of conical plug or "operculum," which serves to close the mouth of the tube when the animal is

Fig. 75.-Cirrhatulus grandis, an "Errant Annelide," in its living condition. (After Verrill.)

retracted within it. In Spirorbis (fig. 74, b) the shelly tube is coiled into a flat spiral, which is fixed to some solid object. It

is of extremely common occurrence on the fronds of sea-weed, and on other submarine objects.

ORDER IV. ERRANTIA.-The Annelides comprised in this order are called "errant" (Lat. erro, I wander), or "roving," from the fact that they all lead a free existence, and are never confined in tubes (fig. 75). They have always lateral unjointed appendages, or foot-tubercles, which carry tufts of bristles, and a soft jointed filament or cirrhus. The anterior rings of the body are usually so modified as to form a sort of head, which is provided with eyes and with two or more feelers, which differ from the antennæ of insects and Crustaceans in not being jointed. The mouth is placed on the inferior surface of the head, and is sometimes furnished with one or more pairs of horny jaws which work from side to side. The upper part of the alimentary canal is muscular, and can be turned inside out, or protruded beyond the true opening of the mouth. The pseudo-hæmal system is well developed, and its contained fluid is mostly red. Respiration is effected by external processes, gills or branchiæ, arranged in tufts placed along the sides or back of the body, and not confined to the immediate neighbourhood of the head, as in Tubicola. The sexes are in different individuals, and the young pass through a metamorphosis.

Amongst the best known and commonest of the Errant Annelides are the common lob-worm (Arenicola piscatorum) of our coasts, which is constantly used by fishermen for bait; and the sea-mice (Aphrodite and Polynoe), some of which attain a large size, and are conspicuous for their iridescent bristles. Other less abundant forms may be readily obtained by searching under stones at low-water.

K

CHAPTER XIII.

ARTHROPODA.

DIVISION II. ARTHROPODA or ARTICULATA.-The members of the sub-kingdom Annulosa comprised under this head are generally known as Articulate animals, or as Arthropoda (Gr. arthros, a joint; and podes, feet). They are all distinguished by the possession of jointed appendages articulated to the body. The body is composed of a series of distinct rings or segments (technically called "somites") arranged longitudinally one behind the other (fig. 76). The skin is more or less completely hardened by a horny deposit of "chitine," with or without lime,

Fig. 76.-Arthropoda. The Sand-hopper, Talitrus locusta, enlarged.

so as to form a resisting shell, to the inner surface of which the muscles are attached. There is consequently no necessity for any internal skeleton. The nervous system in the young of all Articulate animals has its typical form of a chain of ganglia placed along the ventral surface of the body, and traversed in front by the gullet. In the adult, however, this typical state of

the nervous system is often lost or modified. The blood-circulatory system may be absent; but when it is present, it is placed dorsally (fig. 70), and consists of a true blood-system containing corpusculated blood, and furnished with a contractile cavity or heart. Respiration is sometimes effected simply by the general surface of the body, but there are generally special organs, adapted for breathing air either directly or through the medium of water. Jointed appendages are always present, and may be developed from any segment of the body.

The Arthropoda are divided into four great classes—viz., the Crustacea (crabs, lobsters, &c.), the Arachnida (mites, spiders, and scorpions), the Myriapoda (centipedes and galley-worms), and the Insecta (or true insects). These are roughly distinguishable from one another by the following characters :—

I. CRUSTACEA.-Animal more or less truly aquatic; respiration by gills, or by the general surface of the body; two pairs of antennæ (feelers); locomotive appendages more than eight in number, borne by the segments of the thorax, and usually of the abdomen also.

2. ARACHNIDA.—Respiration aerial, by pulmonary sacs, by air-tubes (trachex), or by the general surface of the body; head and thorax amalgamated; antennæ (as such) absent; legs eight; abdomen without jointed appendages.

3. MYRIAPODA.-Respiration by air-tubes (tracheæ); head distinct ; remainder of the body composed of nearly similar segments; one pair of antennæ; legs numerous.

4. INSECTA.-Respiration by air-tubes (trachex); head, thorax, and abdomen distinct; one pair of antennæ; three pairs of legs borne on the thorax; abdomen destitute of limbs; generally two pairs of wings on the thorax.