ANNELIDA
ANNELIDA, a class of the sub-kingdom Annulosa, the _/\_ title being derived from Lamarck s term Annelides (Annellus, a little ring), given to Cuvier s red-blooded worms. The latter were in cluded by Lamams under his Vermes, along with intestinal worms, molluscs, )ophytes, and sponges. The labours of Pallas, Baster, Otho Fabricius, and espe cially 0. F. Miiller, enabled Gmeliu, in the 13th edition of the Systema Naturce of the great Swede, to make some improvements and many additions. Cuvier ranged the Annelida under his Articulata, giving them the chief position on account of their red blood ; but they are now generally classified as a separate type below the latter. It would be out of place to enumerate all the ad vances that have occurred since Cuvier s time, but among those whose names stand prominently forward in this respect are Lamarck, De Blainville, Bosc, Audouin, Milne-Edwards, Duges, Moquin-Tandon, and De Quatrefages in France ; Van Bene- den, Morren, and D Udekem in Belgium ; Gruithuisen, Oken, H. Rathke, Grube, Max Sehultze, Hoffmcister, Hering, Kolliker, Schmarda, Keferstein, Ehlers, and Ratzel in Germany ; Claparede in Switzerland ; Malmgren, Metschnikoff, and Kowalevsky in Russia ; Sars in Norway ; Kinberg and Love"n in Sweden ; Delle Chiaje in Italy ; tStimpson, Leidy, A. Agassiz, and Verrill in North America ; and Montagu, Dalyell, G. Johnston, J. R. Johnson, Williams, Huxley, Baird, and Ray Lankester in our own country. Fm ^_ N(reis ^ a .
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The Annelida may be described as ? < i<. (4/tf bilaterally symmetrical animals, with flat tened or cylindrical bodies, composed of numerous soft rings, or without such. The locomotive appendages (gene rally furnished with bristles) are not articulated. # Nervous system consisting of a cephalic ganglion or pair of ganglia, connected on each side of the oesophagus with a chain of ganglia running along the ventral aspect. Mouth ventral ; ali mentary canal with an anus. Circulatory system with distinct vessels. The majority are small animals, but some reach the length of 6 feet and upwards, and are as thick as a finger.
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In the present article the Annelida will be understood as comprehending the A. Polychæta, A. Oligochæta, A. Onycophora, A. Discophora, and A. Gephyrea, the first two being often placed together in the sub class Chsetopoda.
I. In the first order, A. Polychæta, the body (fig. 1) is elongated, segmented, more or less cylindrical (certain tubicolous forms having two u-ell-marked regions), and almost universally furnished with uniramous or biramous bristle- bearing feet (parapodia, Huxley). The first segment is modified, so as to form a head or snout, and has the eyes, tentacles, &c. The second or buccal segment bears the mouth and Malm/ ^ rmi - certain processes. The other segments have foot-tuber cles, bearing bristles developed in special sacs (e, fig. 4); and various appendages, such as elytra, dorsal and ventral cirri, &c. The structure of the bristles (e.g. fig. 2) holds a prominent part in the discrimi nation of species and even genera, and the same may be said of the hooks of many of the Tubicola (fig. 3). A curious example of the Opheliidai has no bristles, and Tomopteris has only one at the base of each tentacle. Poste riorly, the diminished body gene rally terminates in two elongated cirri in the free forms, while in the sedentary such processes are often absent. In Poly- dora (fig. 21) a curious funnel-like structure exists.
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FIG. 3. Hook of Terebella.
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FIG. 4. Transverse section of Eunice. (After Ehlers 1
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FIG. 5. Ventral vascular system i , ,. i of Nereis cultrifera, Grube (After
finely fibrous, cuticular, or chitinous layer (d), to which the iridescence in many forms is due. This is perforated at somewhat regular intervals by two series of pores, and cilia are generally present at certain points. In the sedentary anne lids the cuticle is thinner, a feature probably in connection with their protective tubes. Be neath the cuticle is a cellulo- granular hypoderm, with tubular glands, bacilliparous and glandu lar follicles in certain cases, as well as the deposits of pigment. The muscular system is well de veloped, and the bands variously arranged in the body-wall. A well-marked circular layer is gene rally present externally, while in others it would seem to be repre sented only by oblique bands. The longitudinal bundles follow different plans, such as forming an almost continuous coat, or split up into various distinct ribands (a and b, fig. 4, the former repre senting one of the dorsal bands, the latter one of the ventral). Schneider thought that the ar rangement of the muscular sys tem might form the basis of a classification of this and allied groups; but for sound reasons j.- Met .^ , proboscis ;a ,cesopha- it has not been adopted. There KUS; a", intestine; b dissepi- r . -i mcnts; v, ventral blood-vessel is no tendinous raphe at eacn
segment, though fibres from the dissepiments pass through