Fourth Order.—ANNELIDA SUCTORIA.
Body destitute of bristles for locomotion, com pletely apodous, and without soft appen dages. A prehensile cavity in the form of a sucker at each extremity of the body.
Head, not distinct, but generally provided with eyes and jaws.
This order is composed of the family of Hiru dinida, and of the genus Branchellion.
External conformation.—The Annelida have always an elongated, generally cylindrical, and vermicular form ; sometimes, however, they are flat or more or less oval. The body is com posed, as we have already observed, of a series of rings, not of a horny or calcareous texture as in the majority of insects and crustacea, but membranous and separated from each other only by a transverse fold of the integument; as is seen in certain larvae. The number of these rings is occasionally very considerable (some nereida have more than 500), and in many annelida it varies considerably in different individuals of the same species, and seems to increase with age. In some instances these segments are sub divided into two or more transverse bands by furrows.
In general each ring supports a pair of mem bers, and when an apparently single segment gives origin to a greater number of these or gans, it is easy to perceive that it results from the union of many rings blended together. The two extremities of the body are sometimes dilated in the form of suckers (in the sucto-rious annelidans), but in general nothing of the kind exists, and the anterior extremity either resem bles the rest of the body, or it terminates in a head more or less distinct (as in the nereida, see fig. 62), often supporting eyes (a), and fili form appendages called antennae, (b, c), the num ber of which is generally three, or five.
The mouth is situated at the extremity of the body, and in the acephalous annelida is di rected forwards, but in the cephalous species this opening is situated below the base of the head. The anus is placed at the opposite ex tremity, and is almost always found on the dorsal aspect of the body. A certain number of Annelida are completely apodous, and do not present the least trace of an appendage on any of the segments of the body (the hirudinidm).
Others exhibit on either side many rows of bristles, which fulfil the office of feet (the terri col). In others, again, the bristles of which we have spoken are supported on a fleshy tubercle more or less prominent, and more or less complicated in structure, and to these organs the name of feet is applied.
The feet of the Annelida, when they present the maximum of development of which they are susceptible in that class of animals, are com posed each of two very distinct portions, placed one above the other, and appertaining the one to the dorsal, the other to the ventral arch of the ring. (See fig. 63,which represents one of the feet of an amphinome.) M. Savigny, who was the first to study with due care the zoological cha racters furnished by these appendages of the annelida, gave to these portions of the feet the names of dorsal oar (a) and ventral oar (b) (rame dorsal et rame ventral). Sometimes these oars are pretty distant from one another, (fig. 63.) sometimes they are separated only by a shallow fissure (fig. 64. which represents the foot of a nereid), and occasionally they are so intimately blended together that they can hardly be dis tinguished, and form, as it were, but a single organ ; lastly, there are cases in which only one of the oars would seem to be developed. If one were disposed to compare the loco motive system of the annelida with that of the other articulate classes, the ventral oar should be regarded as analogous to the members which in the Crustacea, Insects, &c. are variously modified to constitute the legs, the jaws, or the antennae : and the dorsal oar ought to be considered as representing the appendages, which, though wanting in the greater number of articulate animals, yet acquire a considerable develop ment on the last two rings of the thoracic segment of most insects and constitute the wings. In this particular the annelida afford an example of the greatest uniformity in the development of the appendicular system in the articulate division of the animal kingdom.