Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/76

66 the longitudinal layer, in some cases even to the hypoderm. The muscular fibres have a hyaline connective tissue. In Myxicola and Protula the fibres of the longitudinal coat have a dendritic appearance in transverse section. Besides the foregoing, many have a powerful oblique band on each side, passing down to be inserted near the nerve-cord, or elsewhere ; and the proboscis (a, fig. 5), bristles, branchiae, alimentary canal, and other organs, have a series of special muscles. By the muscles of the body-wall the Polychceta execute all their movements in the water and on land, the feet-papillae with their bristles playing quite a subordinate part. A few progress in addition by special organs, such as the tentacles of the Terebelhe. The cutaneous and muscular tissues enclose a space termed the perivisceral cavity, which stretches from the anterior to the posterior extremity. This chamber is lined by a special membrane, which likewise envelops the ali mentary canal, vessels, and other organs. It is generally divided into various transverse spaces by muscular dis sepiments or screens (which permit communication), and occasionally there are longitudinal partitions. In this cavity the highly organised perivisceral corpuscles float in a coagulable lymph, which has no relation with sea-water. The lining membrane is ciliated in Aphrodita, Glycera, Polycirrus, Tomopteris, and Terebella ; and it is curious that in these the blood-vessels proper are absent, with the exception of the first and last. Hay Lankester has found that haemoglobin occurs in the perivisceral corpuscles of Glycera and Capitella. Professor Huxley, after M. de Quatrefages, is inclined to think the perivisceral fluid the true blood of the group, but this is doubtful. Fio 6. DasycTione in- ., ,1, -IT The circulation of the blood is effected by a system of rlosed vessels with muscular Avails (fig. 5), the funda mental arrangement consisting of a dorsal and a ventral trunk (v), with communicating branches in each segment the current in the former being directed forwards, and in the latter backwards. Branches from the main trunks ramify on the wall of the ali mentary canal, body-wall, dissepi ments, and other parts. The dorsal gives branches to the branchiae, whence the blood passes by other twigs to the ventral. Sometimes dilatations, termed " hearts," occur at the bases of the branchiae as in Eunice ; and in Arenicola and Polyophthalmus similar dila- fracta, Kr (After Maim- tationsexist in other parts. Various grm:) modifications of the system take place, such as the bifurcation of the dorsal vessel anteriorly or posteriorly, the presence of blood-sinuses or lacuna? round the intestine in certain sedentary forms, and great diversity in regard to capillary distribution. The blood is usually reddish in colour, and devoid of corpuscles the latter, however, being present in Syllida, some of the Ophe- Iiida3, Cirratulidae, and in Staurocephahis. It is colourless in Aphrodita, greenish in Stylaroides and certain Sabellidte. Finally, Glycera and Polycirrus are devoid of a true circula tory apparatus. This system is compared by Professor Huxley to the water-vascular system of the Echinoderms, Trematoda, and other groups ; but, as formerly mentioned, this is doubtful. Hemoglobin has been found by Ray Lankester in. the blood of many of the Polychasta. The Annelida composing this group generally possess a special branchial apparatus, and where this is absent (e.g. Li(mbriconereis ), the skin and ciliated digestive chamber seem to carry on the function. So characteristic are the branchiae, that the term Dorsibranchs was aptly bestowed on one section (see fig. 4), and Cephalobranchs on the other (see fig. 6). The branchiae have the form of simple filiform organs, or they are pinnate, bipinnate, pectinate (e, fig. 4), flabelliform, or arbuscular processes. To each the blood is carried by a branch of the dorsal vessel, which traverses the organ to its extremity, and, looping, carries the aerated fluid to the ventral vessel. Vascular loops pass between the two branches in the branchiae, and only very thin cuticular tissues covered with cilia intervene between the water and the blood, so that due oxygenation occurs. In the Ser- pulidse the branch from the dorsal vessel is continued directly into the twig going to the ventral, and from their point of union a single vessel traverses the branchia and sends processes into the pinnas, the blood, moreover, in these presenting undulatory movements. A cartilaginous framework exists in the branchial processes of the latter group and the Sabellidae (fig. 6). According to Fritz Miiller the opercular plug in certain forms is developed by a gradual metamorphosis of the branchia. FIG. ?. Tip of proboscis The digestive system consists of a rounded alimentary canal, commencing at the second and terminating in an anus at the last or the penultimate segment. The mouth opens in the form of a simple slit or grooved dimple on the under surface of the second segment. In many the first region is a complex muscular and protrusible pro boscis (fig. 5, a, pharynx and oesophagus of some, and the oesophagus of others), with papillae, horny or dense calcareous teeth (fig. 7), with a narrow ocsophageal region behind. Certain glandular organs in the oesophageal region of certain forms are termed salivary. The walls of the suc ceeding portion (a") are often marked by numerous sacculations from constrictions at the dissepiments; and in some (e.g., ^USSUSw- Aphrodita) large diverticula occur. The ter savigny.-, inner surface bears vibratile cilia, the walls are glandular, and there are also circular and longitudinal muscular fibres. The gland-cells contain numerous refracting granules, and the tissiie is by some held to be homologous with the liver of the higher forms. Many modifications of this system exist : thus, for example, there is in Syllis a firm, smooth anterior region (pharynx), often armed, then a region provided with glands (proventrictdus or gizzard), a stomach with a glandular sac on each side, and lastly the intestine. The food of the group consists of both animal and vegetable substances, and sandy mud is swallowed by others for the nutrient particles it contains. The nervous system has the form of a pair of cephalic ganglia, and a chain of ganglia placed along the ventral aspect of the body (n, fig. 4). The former lie over the oesophagus, and send branches to the eyes, tentacles, and other organs. A cord (buccal) joins them on each side to