Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/81

Rh greatly developed, and iu definite layers, and many swini as freely as the most active leeches. The digestive system intimately agrees, having cilia on the inner surface of the canal, a muscular cesophageal region, and a sacculated intestine, while the glandular and other elements in the wall are very similar; it differs in not passing through a buccal nervous collar, although it lies beneath the nervous system in this region. The circulatory apparatus is fairly developed, and in some corpusculated. The special plexus, in certain forms, in the oesophageal part of the digestive tract points out not only the true function of the vessels (which have been considered a water-vascular system with out reference to the cephalic sacs and their ciliated vessels in the Enopla), but shows a close analogy with the anterior plexuses of many Annelida proper and Balanoylossus, and even foreshadows the branchial system of certain verte brates, as seen, for instance, in the young Petromyzon. The appending of the branchial system to the anterior end of the digestive is characteristic. Though it is quite in accurate to say that the Nemerteans have a corpusculated fluid in the general cavity of their bodies, yet a highly organised corpusculated fluid exists in their muscular proboscidian chamber, and evidently performs important functions in their economy Moreover, the remarkable proboscis and its sheath pass through a ring of nervous tissue consisting of the superior and inferior commissures and their connections with the ganglia. The cephalic ganglia are large, and lie over the digestive system, but the nerve-cords are separated throughout. The Nemer teans are chiefly dioecious, and the products of their sexual organs, which are developed in the form of a series of sacs on each side between the muscular wall of the body and the digestive canal, find exit by lateral pores, the contents of the male organs being often vented in clouds, as in Hermella, The young sometimes undergo a metamorphosis, e.g., the Pylidium-development of certain Anopla. To this the Tomaria-condition of Balanoylossus and the Actinotrocha- state of certain Gephyrea show similar features; and the three forms lead by separate channels to the Annelids proper. There is little analogy with the Tuuicates, but the similarity of the development of certain Echinoderms to the three forms just mentioned is emi nently suggestive.

Finally the Annelida as a whole show certain general features which may be grouped under three heads (1.) The uses of the class to man; (2.) The property of phosphoresence ; and (3.) The power of boring into hard substances. The Annelida are not devoid of value in an economical sense. All round the British and many other coasts the lob worm (Arenicola marina, fig. 20) is used as bait; and here and there Nephthys caeca and Nereilepas fucala. In the Channel and Channel Islands two of the most plentiful of the Nereids (Nereis cultrifera and N. diversicolor) F io Ja are extensively employed in fishing. They are constantly sought for between tide-marks with a pointed instrument resembling a spear, and kept in vessels amongst sand and seaweeds. One of the most esteemed baits in ordinary and in conger fishing in the same regions is the large Marphysa sanguinea. The anterior segments of the living annelid only are preserved, since the posterior region is apt to decay and cause the death of the whole. The natives of the Fiji group much relish a form allied to our Lysidice ninetta, and they predict its annual appearance in their seas by observing the phases of the moon. It is called Palolo by the Samoans and Tonguese, and Mbalolo by the Fijians. Occurring in vast numbers, formal presents of the esteemed food are sent by the fortunate chiefs con siderable distances to those whose dominions are not visited by the annelid. If the latter has similar habits to the British Lysidice, it probably leaves its retreats in the coral- reefs and rocks for the purposes of reproduction. The extensive use of the Lumbrici in fresh-water fishing, and that of the leech in medicine, need only be alluded to the latter forming a considerable item in British importa tions. Echiurus is employed as bait by the Belgian fishermen, and Pallas records that the natives of the same- coast formerly considered the muscular proboscis of the sea-mouse good food. .Lastly, a Sipunculus is eaten by the Chinese. An examination, again, of the stomachs of our most valuable fishes shows the important part played by the Annelida in their food-supply; and the large number of species of fish which can be speedily captured on a rich coast with bait of Nereis cultrifera is ample cor- roboratiou. The stomachs of cod and haddock, for instance, are often quite filled with sea-mice with Polynoidae, Tere- bellidas, Alitta virens, Oivenia, Troplionia, Phascolosoma ; and in fresh water those of trout with Lumbrici. The property of phosphorescence occurs in the families Polynoida3, Syllidre, Chastopterida?, and in Polycirrus and Lumbricus. In the first-mentioned, light greenish and somewhat steady scintillations are given off at the attach ment of each scale, and the separate organ gleams with pulsations of light at the ruptured surface. It was pro bably the latter appearance which caused M. de Quatre- fages to state that it was emitted in muscular contraction. The synchronous emissions of light by the Italian fire-flies is interesting in this respect. In the Syllidse the light comes from the under surface of each foot; in Gh(Ktopterus the most vivid luminosity is on the dor- suni of the tenth seg ment ; and Polycin-us is so phosphorescent, that the slightest tremor - in the water causes vivid pale bluish fire to gleam along every tentacle. The exact physiology of * the luminosity is still open to investigation, though P. Panceri of Naples con nects it with certain fatty granules. The luminous emissions have no con nection with light or darkness, with the cap ture of prey, or the alluring of their enemies, nor with the illumination of the depths of the sea. Fio. 21. Pandora ciliala, Johnst. Boring and burrowing in sand, mud, and earth are very general in the Annelida. Glycera and Ne2)hthys, for instance, disappear with great rapidity amongst sand by boring with their proboscides, the former passing its elon gated organ through a considerable space in a single thrust. Eteone and Ammotrypane carry their bodies swiftly