'CHAPTER XIII. OF THE VERMIFORM CLASSES. 831. We are now arrived at the lower division of the Articulated Sub-kingdom, in which there is an absence of articulated members, and a general inferiority in the structure of the animals composing it; so that, as we descend, we lose one after another of those characters, by which this group is distinguished. Nevertheless, the lateral symmetry (§ 47) of the body is almost uniformly preserved; and it is only in the very lowest that we meet with any approach to the circular arrangement of the parts of the body, which is characteristic of the Radiata. The Nervous System cannot be traced in the simplest animals of this division; but wherever it can be detected, it presents the same essential characters as in the higher classes,-consisting of a double cord running along the ventral surface from one extremity of the body to the other, and studded with ganglia at intervals. These ganglia are smaller, in proportion as they are more numerous; and their size diminishes, too, with the diminution of the locomotive powers of the animal. The cephalic ganglia, or those which are placed at the anterior extremity of the body, above the œsophagus, are usually larger than the rest; especially when they are connected with organs of special sense, such as eyes and antennæ. But as we descend through this series, we find the eyes disappearing, and then the antennæ; so that the head is only marked by its being the situation of the mouth; and the cephalic ganglia are then scarcely to be distinguished from the rest. $32. The body of these animals is generally long, slender, and more or less cylindrical; it is frequently divided into distinct segments; but these are only marked externally by a folding or wrinkling of the integuments; and there does not GENERAL CHARACTERS OF VERMIFORM ARTICULATA. 297 exist any proper tegumentary skeleton or hard envelope, as in most of the higher classes of Articulata. In the lowest mem FIG. 519.-LEECH. bers of the class, indeed, we cannot even trace an integument distinct from the contained tissues; all being alike soft, and sometimes almost jelly-like. Even here, however, the division into segments is most distinctly marked, by the repetition, in each segment, of nearly all the organs of the body; so that the animal is much more capable of sustaining severe injuries, than is the case in the higher classes; and it has also greater power of repairing them, new segments being developed to replace those which had been lost. The greater number of the Vermiform or Worm-like Articulata are aquatic in their habits, living either in water or in moist situations; and as a general rule, their respiration is performed either by gills, or by the general surface of the body. They have usually a distinct circulating apparatus; which serves not only to convey the nutritious fluid through the several tissues, to whose growth it is to contribute, but also to carry it to the organs in which it is to undergo aeration. This apparatus consists, as in Insects, of a dorsal vessel, running along the greater part of the body, and contracting from behind forwards, so as to expel the blood through the branches which proceed from its anterior extremity; the fluid then flows backwards through the body, affording nutriment to its several parts in its course; and it is then made to flow over the walls of the alimentary canal, so as to take up from its cavity any new materials, which may have been prepared for it. The provisions for carrying it through the respiratory organs vary greatly, in proportion to the variety in the arrangement of the respiratory organs themselves, which we shall see to be great, as it is in the Mollusca: but there is not unfrequently a distinct contractile cavity, or respiratory heart (ANIM. PHYSIOL. § 281), for propelling the blood through every pair of gills, as in the 298 VERMIFORM ARTICULATA.-CLASS OF ANNELIDA. Crustacea (§ 777). In the lowest Articulata, however, no dorsal vessel, or impelling organ of any kind, can be discovered; and the circulating system consists merely of a network of capillary vessels, occasionally merging into larger trunks; the blood being propelled through it by the continual movements of the body itself, which act upon it in somewhat the same manner that our own changes in position do upon our venous circulation (ANIM. PHYSIOL. § 279). 833. This group may be regarded as especially represented by the class ANNELIDA; which contains, with the Leech and Earthworm, a great number of marine animals less generally known. In the ENTOZOA, we find the characters of the Articulated series in their lowest state of development. And in the ROTIFERA, we meet with peculiarities of form so great, that the affinity of the minute animals it contains to the preceding classes, is by no means apparent without a careful examination. CLASS OF ANNELIDA. 834. The Annelida may be characterised as possessing an elongated body, marked by transverse lines that divide it into numerous segments: and usually furnished with a series of locomotive appendages in the form of bristles, sometimes implanted on fleshy tubercles; but not with articulated members. They have a complete apparatus for circulation and respiration; and FIG. 520.-NEREIS. the ventral chain of nervous ganglia may always be distinguished. Of the appendages, which often combine the functions of loco LOCOMOTIVE APPENDAGES OF ANNELIDA. 299 motion and respiration, there are frequently two pairs in each segment; one belonging to its upper or dorsal portion, and the other to its lower or central (Fig. 594) but in other instances the two appendages on the same side are united (Fig. 527). We usually then find, in the marine Annelida, that the ventral portion is chiefly devoted to locomotion; and the dorsal to respiration. Thus, in the Eunice, we find, at the under part, a fleshy tubercle (t), furnished with a tuft of bristles, and below it a rudimentary cirrhus, or tendril-like organ (i); whilst the upper part of the appendage is formed. by a branchial tuft (), and by a long slender cirrhus (c). This last sometimes exhibits a trace of articulation, as in the Syllis (Fig. 522, a). In other cases, however, these appen t FIG. 521.-GILL-TUFT OF FIG. 522.-SYLLIS MONILARIS, with one of its locomotive organs and setigerous appendage attached thereto. dages are only represented by a few short stiff hairs, as in the Earthworm; and in other instances, as the Leech, there is no trace of any members or appendages to the body. The bristly tufts of the Nereidans and their allies are useful to them in various ways; they serve them in part as instruments of attack and defence, the bristles being usually sharp, and sometimes barbed at their extremities, so as to attach themselves with force to soft substances; they assist, also, in their movements over solid surfaces, taking hold, as it were, of the rock on which the animal is crawling, so that the hinder part of the body is prevented from slipping back, when the anterior part is pushed forwards; and they also aid in its movements through the Giz 524. 300 GENERAL CHARACTERS OF ANNELIDA. water, serving in some degree as oars by which it is propelled. In some instances, indeed, we find the tufts replaced by flattened plates, which are specially adapted for this last purpose. Where there are no locomotive appendages, the extremities of the body are usually furnished with suckers, which give important assistance in locomotion,-as in the well-known Leech. But in one tribe of this class, the animal, in its adult form at least, enjoys very little power of locomotion, being confined within a shell, which it constructs for itself, and which is attached to some solid support. 835. The first segment, which constitutes the head, is usually provided with one or more pairs of imperfectly-formed eyes; and also with several appendages analogous to the cirrhi of the other segments, which are considered as antennæ or tentacular cirrhi. The mouth, which is situated on the lower side of the head, is constructed on a very different plan in the several divisions of the class: being sometimes furnished with one, two, or three pairs of hard horny jaws, with toothed or pointed edges; sometimes having a sort of trunk, which can be pushed out or m b m tr FIG. 523-HEAD AND TRUNK OF GLYCERIS; C, anterior portion of the body; t, head; tr, trunk; b, opening of the mouth; m, m, jaws. drawn in (Fig. 523), and which bears a pair of small tooth-like t jaws at its extremity; and sometimes being situated in the centre of a flattened sucker, and armed with an apparatus of little saws, as in the Leech (Fig. 532). The alimentary canal is usually simple in its form, passing in a straight line from one end of the body to the other; and not exhibiting any distinction of stomach or intestine. It is often furnished with a number of little saccular appendages, placed along the greater part of its length; these are probably secreting organs, as we do not find any others which can be regarded as having that character. Many of the Annelida are remarkable for the red colour of their blood; this colour is not given, however, by red corpuscles, but exists in the liquor sanguinis (ANIM. PHYSIOL. 229). Sometimes, however, the blood has rather a |