Each oar is essentially composed of a fleshy tubercle more or less prominent, which sup ports different productions of the integument, incloses the bristles (c), and which is more especially designated by the name of foot. Towards the base of the setiferous tubercle there is generally a membranous appendage, sometimes filiform, sometimes lamelliform, called the cirrus (d, e); lastly, it is also above the margin and near the base of these organs that the branchiw (f) take their origin, but in general it is only the dorsal oar that supports them. All the above parts may exist simul taneously, but it often happens that one or more are atrophied to a greater or less degree, or are altogether deficient; and this either along the entire body or on certain segments only. Thus in the terricolous annelida there are no cirri ; in the hermellw they are pre sent on the ventral, but not on the dorsal oar; while in the cirrhatuke the reverse obtains.
In most of the annelida errantia the setiferous tubercle of both oars is wanting on the first rings which follow the head, whilst the cirri assume a very great development, and form the appendages termed by systematic authors ten tacular cirri. ( Fig. 62, d.) A similar modification may be frequently remarked in the composition of the appen dicular system of the last ring of the body, and thence results a certain number of filiform pro ductions called styles. Lastly, the antennae of the annelida, which must not be confounded with the antennae of insects and crustacea, may also be considered as representing the cirri of the dorsal oar of those rings, the union of which constitutes the head.* The annelida pass in general a somewhat stationary life, and a great number among them remain constantly buried in the earth or enclosed in tubes formed by the mucus which is secreted by the skin, and which, while hard ening, commonly agglutinates together frag ments of shells and sand. The formation of these sheaths is very quick. I have seen them fabricated in the course of a few hours. Some times they are of extreme tenuity, occasionally they are as tough as thick leather, and there are some which possess very considerable hardness and are composed in great proportion of carbonate of lime, like the shells of mol lusca. In the greater part of these animals locomotion is produced by general undulations of the body determined by contractions of a layer of muscular fibres extending from one ring to another, and fixed to the inner surface of the skin. But in other species the change of place is effected by the action of the feet, of which we have spoken ; or by the contrac tion of the tentaculT which surround the mouth, as in the terebelloe, and which, by shortening themselves, drag on the body of the animal in the same manner as the arms of the cephalopods : lastly, by the action of the suckers with which the extremities of the body are furnished.
The bristles (fig. 63 and 64, c,) with which the feet of the annelida are provided, do not serve merely as little levers to facilitate their move ments, but are also offensive arms, and their structure is very curious. They differ con siderably from the hairs of other articulate animals, which are nothing more than small tubular prolongations of the epidermic layer. By their mode of connexion with the integu ments and their mode of formation they ap pear to approach the hair of mammalia, but their disposition is of a more complicated na ture. They are inclosed in sheaths provided with muscular fibres, by the aid of which the animal can protrude and retract them again : in general, also, they are not merely simple conical filaments, but their extremity is often shaped like a harpoon, a lance, or a barbed arrow, and the annelidan uses it to inflict a wound upon its enemies.* Sensation.—Tactile sensibility is considerable in these animals, and it seems to reside prin cipally in the antennae, the cirri, and the tentacula. They do not appear to possess a sense of hearing, and there are many among them which do not manifest any sign of seri sibility to light ; but in others, eyes (fig. 62, a,) exist, the number of which is sometimes very considerable, but the structure very simple.
They are coloured points, (generally black,) and situated on the dorsal aspect of the head or on the cephalic sucker. In the setiferous anne lida there are never more than two pairs, but in the hirudinidm or leeches their number often increases to eight or ten. The anatomy of these eyes has recently been studied by Professor Muller of Berlin, and according to his researches it would seem that these organs do not contain a crystalline lens, or a trans parent body analogous to the vitreous cones of the crustacea and insecta, but consist simply of a terminal ganglion of the optic nerve covered by a layer of black pigment and placed imme diately beneath the integument, which is thin and transparent at that part.* Nervous system.—In like manner the ner vous system of the annelida is very simple. It occupies the middle line of the ventral aspect of the body, and con sists of a double series of mi nute ganglions of medullary matter, more or less inti mately united or even blended together, and equal in number to the number of rings. (Seelig. 65. repre senting the nervous system of the aphrodita aculeata). The ganglions give origin to lateral branches, and are connected together by two chords of communi cation, sometimes separate, sometimes united into a sin gle trunk, so as to constitute a longitudinal chain extended through the entire length of the body. The first of these ganglions (a) is lodged in the head, or at least at the ante rior extremity of the animal, in front of or above the di gestive tube ; the rest are placed below that canal ; whence it results that the two nervous chords which form the media of communi cation between the cephalic ganglion and the first of the sub-oesophageal series pass along the sides of the oeso phagus, and form around that canal a species of collar or ring ; a character which is common to all the articulate animals.f Organs of digestion.—The alimentary canal in the annelida extends from one end of the body to the other, and has an external com munication at both extremities. The mouth is generally provided with a projectile proboscis, which is formed by the anterior portion of the digestive canal, which can be inverted and pro truded like the finger of a glove, and possesses muscles for the express object of effecting these movements (see fig. 66, which represents the proboscis of a phyllodoce, and fig. 67, that of a nereis). The surface is frequently beset with small and its extremity armed with horny jaws (m), the disposition of which varies in different genera. It is to be observed that these jaws are almost always placed laterally like the mandibles of other articulate animals, and cannot act upon one another in the direc tion of the axis of the body, as in the vertebrata, but are not to be regarded as analogous to the mandibles and maxillae of insects and crustacea. In their structure, the jaws of the annelida ap proximate rather to the solid plates with which the interior of the stomach in some crustacea is provided, and to the hooks which arm the mouth of certain gasteropodous molluscs. This conformation of the oral apparatus is met with only in the annelida errantia ; in the annelida terricola there is scarcely a vestige of a proboscis, and never any teeth or jaws. In the annelida suctoria, the mouth, which is placed at the bottom of the cephalic sucker, is also occasionally protruded in the form of a small tubular proboscis, and in other species its margins are armed with little horny jaws ; lastly, in the annelida tubicola, nothing of the kind is to be seen, but in general the superior border of the mouth forms a sort of projecting lip, which is provided with long tentacles, sometimes simple and filiform, sometimes pec tinated and resembling tufts. In certain erratic annelida, the Agliope, for example, there are also found around the mouth small tentacula, which are quite distinct from the tentacular cirri, and which appear to be analogous to the appendages of which we have just made mention.