a comparison of them with the stigmata of insects, and they have been even mistaken for true respiratory spiracles; but it would be more correct to compare them with the hooked bristles that garnish the prolegs of caterpillars, and are subservient to progressive motion. In most Annelides the feet are essentially the same in structure throughout the body. In those which have hooked bristles, the feet furnished with them are found only on a certain portion of the segments, and the feet of the other rings are armed with bristles properly so called.

No. IX.

6. Cirri. The cirri have often the form of long subulate fleshy tentacles (No.VIII. figs. 3, 4, 5, e, f), but sometimes they are merely small compressed leaflets or papillary tubercles. In general they are partially contractile; and some have a ringed or moniliform appearance. The cirri of the anterior segments, in which the feet are obsolete or wanting, are, as has been already mentioned, called tentacular cirri, or simply tentacula. The anal segment is equally apodous, and the cirri which issue from its end are named styles (No. IX. fig. 6, a).

6

The proper cirri are usually two to each foot, originating near the base of each branch, and hence they are distinguished into the superior (figs. 3, 4, 5, e) and the inferior cirrus (f). The cirrus may arise, however, from the apex of the dorsal branch; and there are Annelides in which so many as seven pairs are to be counted on a single ring; but in these instances they represent all the other fleshy appendages. The cirri are almost always absent in the Annelides whose feet are coronated with hooked bristles; and also from those whose feet consist merely of a few bristles without any protuberant papillary base.

7. Branchiæ.-The structure of the organs so designated permits, in some instances, no doubt to be entertained as to the functions they are destined to fulfil. Such is the case when the organ is in the form of a little crimson arbuscle, or in tufts or fan-shaped crests (fig. 9); but when it is a simple or a pectinated filament (No. X. figs. 7 & 8), or a nipple-like lobe, or merely a membranous tubercle, there is no organic character to distinguish them from cirri, and their function is less decided*. In number and position the branchiæ are as variable as in their figure, and they are placed either on the head, or along the back, or along the sides, or at the tail. M. de Quatrefages arrives at the following conclusions as to the respiration of the Annelides:"1. Respiration general and entirely cutaneous (Lumbriconereis).-2. Respiration cutaneous, but confined to certain segments (Chatopterus).-3. Respiration cutaneous, but confined to certain points of each segment (Nereis).-4. Respiratory organ taking the form of a simple cæcum or bladder into which the blood

The

It is true in general that the appendages of the anterior rings tend to take a dorsal position, and those of the posterior rings a ventral position. branchiæ are always superior to the other appendages of the foot, or dorsad; and, in doubtful cases, the nature of the organ may be presumed from the position it occupies.

flows (Glycera).-5. The branchia is characterized more and more by the formation of a canal in communication with larger or smaller

No. X.

8

7

lacunæ.-6. These branchia may be distributed all along the body (Eunice sanguinea).-7. They may be confined to a certain number of segments placed towards the middle of the body (Arenicola, Hermella).-8. They may all be placed at the extremity of the body so as to form a double tuft (Serpula) *.”

8. Scales.-The scales are found only in a few genera. They have always a dorsal position, and seem to occupy the place of the superior pair of branchial appendages rather than the superior pair of cirri. Their texture is softish, and the margin is sometimes partially fringed with short filaments or fleshy cilia (No. XI. fig. 10a). In general the scales are placed over only such feet as are destitute of cirri, and alternate with these appendages,-an arrangement which has suggested the theory of their being modifications of the latter; but the genus Sigalion offers an exception, for here the two organs coexist on one and the same foot, and supplies a fact to disprove the opinion of their being the analogues of the superior cirrus.

9. Suckers.-There is no instance of a species of Polypodous Annelid with a sucker either at the anterior or posterior extremity.

* Ann. & Mag. Nat. Hist. Ser. 2. ix. p. 155. See also Williams in lib. cit. xii. p. 393.

+ Called elytra by Savigny, whose nomenclature is followed by Audouin and M.-Edwards, but the term is very unsuitable. "Savigny is of opinion that certain dorsal scales, in pairs, observable in two of the genera of his first family of Nerëideans, are analogous to the elytra and wings of insects: this he infers from characters connected with their insertion, dorsal position, substance and structure, but not with their uses and functions; for, as he also states, they are evidently a species of vesicle, communicating by a pedicle with the interior of the body, which, in the laying season, is filled with eggs, a circumstance in which they agree with the egg-pouches of the Entomostracans; and, therefore, Baron Cuvier's opinion, that there is little foundation for the application of this term to these organs, seems to me correct."-Kirby's Bridgew. Treat. ii. p. 145.

It is true, that the body of the Clymene, a genus furnished with feet, is terminated posteriorly by a sort of concave membranous disk, in

No. XI.

a

10

the centre of which the anus opens; but it is ascertained that that disk, though dilatable, cannot execute the office of a sucker.

10. Anus.-The anus is inclined to the dorsal surface and looks upwards. Dr. Williams says-"There exist other minor families of Annelids in which the terminal outlet of the alimentary system is not seated at the extreme end of the body, but at a point, at the side, more or less removed backwards from the head, resembling intimately the pattern on which that of the Sipunculidæ is formed*."

The Annelides rapacia are the most complicated in organization of any of their class. As the ordinal name implies, they are a wandering tribe, and the locomotive apparatus is so much developed, that the species are enabled to creep with considerable quickness, and even to swim with facility. Some of them, as certain Amphi nomes, appear to be truly pelagian, and are only met with in the high seas; but the great majority dwell between tide-marks on the shore, where they find refuge and concealment under stones, or among corallines and sea-weeds. A small number burrow in the sand (Nephthys, Arenicola, &c.), in which they form a sort of sheath by a glairy secretion from the skin; and a few are truly tubicolous; but these furrows are not indispensable, and can be occasionally abandoned without inconvenience. The greater number of the Order are provided with special organs of sense considerably developed; and they have organs of defence in the shape of bristles of curious structure. All of them are inhabitants of the sea or its shore, and only a very few trespass on tidal rivers, not fearing the influence of brackish water. None of them are parasites. Some species may frequently be observed watching in patient solitude, and ready to launch upon weaker worms, mollusks, or other animals on which they prey; but none offers us any instance of the singular instinctive contrivances which surprise us in the study of insects. Several of

* Rep. Brit. Assoc. 1851, p. 220.

them lose with impunity their exterior organs, and even considerable portions of the body, reproducing the lost parts in a short time under favouring circumstances. The anterior half will reproduce the posterior, and that will return the favour by restoring the anterior with the head and all its organs. But if the body is cut into several pieces, it has not been proved that the separated pieces can live, and become each a perfect worm.

In general the Annelides rapacia have a serpentine or myriapod form, but some are oblong or oval. The body is almost always terminated anteriorly by a distinctly defined head furnished, with few exceptions, with eyes, and with antennæ,-a collocation of parts which is not met with in any other order of the class. Underneath the head, and at its junction with the first ring of the body, we find the mouth (No. X. fig. 7a), which is, in some cases, prolonged backwards and buttressed by the feet. The proboscis is composed of one or two rings; it is very often armed with jaws, and not unfrequently the orifice is encircled with papillary filaments (No. VII. fig. 2).

There is a series of feet along each side of the body, having the form of fleshy tubercles, more or less protuberant. These organs are sometimes simple or uniramous,-sometimes bifid, with a dorsal and a ventral branch. The apex of the branch is always the outlet of a brush of bristles, which can be thrust out, and again withdrawn, to a certain extent at least, into the ring, by certain muscles appropriated to that use. These bristles are sharp, rather firm organs, very variable in regard of figure and conformation in different genera, and each brush has a spine in its centre. In only a very few of this Order do hooked bristles occur, and when present they make a part of the ventral branch of all the feet; while in the Tubicolæ this uniformity of structure is not observable,-nor in the Tubicolæ is their presence coincident with the existence of cirri.

The soft appendages of the Annelides rapacia are, in general, numerous and well developed. The cirri are the most constantly present, disappearing indeed only in a small number of genera placed upon the limits of the Order; but among the Tubicolæ the Hermellæ alone have them. Their usual form is that of a filament tapered gently to a point, and more or less contractile; but in some species the cirri are compressed and expanded into leaflets or miniature fins. With few exceptions there are two cirri to each foot (No. VIII. figs. 3, 4, 5).

The appendages which have received the name of Branchiæ are often absent. In some genera they appear under the guise of mere tubercles or fleshy papilla, attached either at the summit or near the base of the foot; in others they are pectinated filaments; and in others much-divided arbuscular tufts of a crimson colour. The scales, which cover the back like a series of tiles, are peculiar to a few genera of this Order.

These soft appendages are repeated in nearly unvaried uniformity along each side of the body, so that every segment is the same in its gear; but in some species the superior cirri, the scales, or the branchial tubercles, appear and disappear alternately from ring to ring.

It is very common to find the first segment provided only with tentacular cirri, destitute alike of setigerous tubercles and proper branchiæ; and towards both extremities of the body its appendages are either obsolete or less developed than they are near the middle. The anal segment has its appendages commonly reduced to a pair of terminal styles, which originate from each side of its aperture (fig. 6).

Such are the most remarkable features in the external anatomy of these annelidans; but the bristles form so marked a character in them, and influence their habits so much, that it seems necessary to enter at length into a description of their peculiarities,—the more especially as they constitute the principal means of defence against the enemies that surround them. Some annelidans, when alarmed, such as the Aphrodites, instinctively contract the body into a sort of ball, bringing the head and tail into near contact; and in this manner seem anxious to elude the danger. Others, that have a long vermiform body, wriggle and twist themselves with violence, as if they would frighten away the foe by their contortions; but these are weak resources in comparison with the protection they derive from their panoply of spines and bristles. And it is curious to remark that as these diminish in size and piquancy, and become better adapted to locomotion than offence, the worms, as if conscious of their defenceless state, forsake the nomade life, and seek additional security by digging themselves furrows for retreat in the sand and mud.

Savigny was, perhaps, the first to notice that the bristles* are generally capable of being retracted within the body of the worm, and of being again protruded at pleasure by means of appropriate muscles; but he does not appear to have studied these organs in relation to their office of defence, nor has he given any figures sufficiently magnified to show how aptly they are fitted for this purpose. It was left to Audouin and Milne-Edwards to fill up this blank in the history of the Annelides, and we proceed to give their analysis of these

organs.

The Bristles are distinguished into spines (aciculi), and bristles properly so called (festuca).

No. XII.

α

The Spine is of one kind only (No. XII. fig. 11). It is distinguished from the bristles by its central position and its larger size. It is straight, conical, pointed, of a brown or blackish colour, and always a shade darker than that of the bristles with which it is associated. It is very rare to find more than one in each branch of the foot, and there are never more than two. Sometimes none can be discovered. The worm, say our authors, employs this weapon to give the "coup de grace" to the enemy against which it is directed, the comparative shortness and stoutness permitting it to be used with a force and precision which it is impossible to impart to the weak and flexible bristles; and the comparatively blunt point making a wide wound. Each spine is composed of the shaft (a),

* See Oersted's definition of the Bristles in his Annul. Dan. Consp.' p. 5, and

copy.