the edges of which are serrated with numerous minute teeth. These three jaws are so disposed as to meet in a single point, in a triradiate manner, and they are the organs by which the Leech cuts through the skin, in order to get at the blood-vessels beneath. Hence the bite of a leech consists of three little cuts radiating from a point.

The mouth opens by a short gullet into an alimentary canal, the first portion of which, usually termed the "stomach," occupies nearly the whole of the body-cavity, and is furnished with eleven membranous pouches on each side. The pouched "stomach" has the function of separating the watery part of the blood which the animal takes as food; and it opens into an intestine which terminates at a distinct vent placed on the back a little in front of the hinder sucker.

There is no distinct heart, and the place of the bloodvessels is taken by a peculiar system of tubes, containing a fluid which appears to play the same part in the economy of the animal as does the true blood of higher organisms.

It is doubtful, also, if there are any definite breathingorgans; but the animal is furnished with a series of little pouches (fig. 11, B, rr) which are placed on each side of the body, and have commonly been called "respiratory sacs." There are seventeen of these little organs on each side of the body, and they open on the lower surface by a series of minute pores.

The nervous system has the form of a chain of little nervous masses (fig. 11, A, n n) placed along the lower surface of the body, and united lengthways by cords. The first of these masses, representing the brain, is placed above the gullet, which is thus embraced by the cords which unite this with the next nervous mass in the series. The top of the head (fig. 11, A, h) also carries ten eyes, disposed in the shape of a horse-shoe.

The Leech attains a length of from two to three inches; its back is olive-green with rusty-red longitudinal stripes, and its lower surface is greenish-yellow spotted with black. In the nearly-allied Sanguisuga medicinalis, the lower surface of the body is olive-green, and is not spotted.

The Medicinal Leech is exclusively an inhabitant of fresh water, and is mostly imported into Britain from Hungary, Bohemia, Russia, and France.

RECAPITULATION OF ESSENTIAL CHARACTERS. — The body is elongated, and is ringed with transverse folds. When lateral appendages are present, these are never distinctly jointed or articulated to the body. The nervous system has the form of a chain of nervous masses placed along the lower surface of the body, and united by longitudinal cords. There is no distinct heart, but the circulatory system of the higher animals is represented, so far as its function is concerned, by a peculiar system of contractile tubes containing a coloured fluid. These characters distinguish the class of the Annelida as a whole.

CHAPTER IX.

CLASS CRUSTACEA.

THE class Crustacea comprises all the animals which are commonly known as Crabs, Lobsters, Shrimps, Prawns, Wood-lice, Water-fleas, and the like, all of which have the body enclosed in a more or less resistant shell. Hence they are sometimes erroneously spoken of as "Shell-fish ;" and hence also their scientific name (Latin, crusta, a crust or shell). An admirable example of this class may be found in the common Lobster (Homarus vulgaris) of British seas.

The Lobster is almost completely enclosed in a strong and hard shell, which is really formed out of the skin, by the deposition in it of lime and horny matter. A little investigation also readily enables us to perceive that the animal (fig. 12) is really composed of a number of rings or distinct segments, placed one behind the other. Theoretically each segment is distinct, and each is constructed upon a similar plan or model. In theory, namely, each

Fig. 12. The common Lobster (Homarus vulgaris), viewed from below. a The lesser antennæ; a' The greater antennæ; n The last pair of footjaws; c The great claws, or first pair of legs; d, e, f, g The last four pairs of walking legs; h, i, j, k, l, m The six pairs of abdominal appendages, the last five being "swimmerets," and the last of all being greatly expanded; t The last segment of the body, without appendages.

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segment consists of an upper and a lower arch of shell, joined to one another at the sides, and sending downwards a plate from the point where they join (fig. 13, 2). In theory, also, each segment carries a pair of appendages," which ought to be composed of two branches springing from a common base. În some cases, as in the tail ("abdomen") of the Lobster, the actually existing segment really does conform to this theoretical type; but in other cases the segment may be very variously modified, and the appendages of the segment, in particular, assume the most different forms in different parts of the body. In practice, also, some of the segments are so amalgamated and consolidated with one another as to render their recognition a matter of great difficulty.

It will, however, greatly help us in studying the Lobster to remember that the body of the animal is really composed of a number of segments (twenty-one), each of which is constructed upon the theoretical plan or type just mentioned, and each of which may carry a single pair of appendages of some kind or other.

If we look at the Lobster from above (fig. 13, 1), we see at once that the body is very plainly divided into two parts, which would familiarly be called the "head" and "tail." The "head," as we should call it, is in truth composed of fourteen rings all amalgamated together, and covered above by a great shield or buckler (ca). The first seven of these segments belong to what is actually the head, whilst the hinder segments belong to what is properly called the trunk or "thorax" (Greek, thorax, a breastplate); and the head-shield shows this division by a transverse groove on its upper surface. On the other hand, the so-called "tail" is composed of a number (seven) of quite distinct rings or segments, which are movably jointed together, and which collectively constitute what is termed the "abdomen" (Latin, abdo, I conceal).

The Lobster is therefore really composed of twenty-one rings or segments, seven of which are free and movable, and constitute the abdomen, whilst the remaining fourteen are amalgamated together, but really belong, seven to a thorax and seven to a true head.

Fig. 13.-1. Lobster with all the appendages except the last pair of swimmerets removed, and the segments of the abdomen slightly separated from one another. ca The great shield covering the head; t The last segment of the abdomen left in connection with the last segment but one. 2. One of the segments of the abdomen, showing the typical structure of a segment and of the appendages; t Upper arch of the segment; & Lower arch; p Plate prolonged downwards from the line where the upper and lower arches unite; a Base of the appendage; b and c Outer and inner branches of the appendage. 3. One of the last pair of foot-jaws, carrying a gill (g).