Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/80

not always correspond in number with, the rings. Other nerves occur in several, e.g., the leech, which has a trunk running along the dorsal surface of the digestive canal. Eye-specks are present in some, and touch is generally much developed. The cupuliforrn organs, in the cutane ous tissues of the head and anterior region in certain

leeches are supposed to be connected with the latter sense.

The Discophora are hermaphrodite (with the exception of the Malacobdellidce and HistriobdelUdce), with the male organs ranged along the ventral surface in the form of a series of testes, which are connected with a common vas deferens leading into a vesicula seminalis on each side. The latter conveys the fluid into the iutroniittent organ, and its secretion agglutinates the spermatozoa into a seminal rope or spermatophore. The female organs consist of two ovaries leading to a common duct, opening into the vagina, which receives the spermatophore. The segmental organs are present, but not in direct communication with the reproductive structures, nor do they carry the products to the exterior, since they are often ceecal. The foregoing diverges from the arrangement in the other Annelida, since the generative products are not extruded into a perivisceral cavity, but pass outwards by a particular apparatus. The ova are deposited in capsules formed by special secretions, those of the leech being called cocoons. No metamor phosis occurs in the development of the young, which by- and-by, in some cases, attach themselves in crowds to the abdominal surface of the parent.

FIG. IS.Hirudo medicinalis, L., var. ojficinalis. The green leech.

The Discophora do not possess much power of regenerat ing lost parts. They are distributed everywhere, chiefly in fresh water and moist places; some are marine. A few attain a large size, e.g., a species from Valdivia is described as being two and a half feet long. Many are ectoparasitic, living on fishes, crabs, and even in jelly- fishes (Bolinidse). In swampy ground in tropical countries certain leeches are often troublesome to travellers. Fossil leeches have been found in the litho graphic stones of Germany They Lave been grouped into five families, viz., Malacobdellidae, Histriobdellida3, Acanthobdellidse, Branchiobdellida3, and KhynchobdellidaB the latter including the medicinal leech of Europe, the green leech (fig. 18), the Bdella nilotica of Savigny (fig. 19), the horse-leech, skate-leech, pond-leech, which is devoid of an anterior sucker, and other well-known forms.

Flo . i 9 ._ s - lv - (After Savigny.)

V. The A. Gephyrea seem to approach the Echinoderms through the Holothuroidea. The body is more or less cylindrical, and, though corrugated, is not definitely seg mented. There is generally a protrusible proboscis, having the mouth at the end or at its base; and the anus is terminal or dorsal.

The cuticle is chitinous, has numerous processes of similar composition, longitudinal and transverse rugaa, and many pores. Beneath is a hypoderm containing certain glandular organs or sacs, and, in some, bodies like tricho- cysts. Bristles occur in EcJiiurus and Bonelha. The muscular system consists of external circular and internal longitudinal fibres, and special groups of retractor and other muscles of the proboscis. In some the longitudinal layer is arranged in separate bands, stretching from one end of the body to the other. The circulatory system shows a dorsal and ventral vessel, both in the Sipunculidas communicating with a circular vessel (ciliated internally) surrounding the oesophagus, and sending prolongations into the ciliated tentacles. The latter contains a corpus- culated fluid. Certain ciliated infundibuliform organs also occur on the intestinal mesentery of Sipunculus, and are thought to be connected with the so-called water-vascular system. In Echiurus there is a more distinct circulation, consisting, according to De Quatrefages, of three longi tudinal trunks a dorsal, ventral, and intestinal. The perivisceral cavity is large, with rudimentary dissepiments in some, and contains a corpusculated fluid, which in the living animal shows very lively currents most marked posteriorly, and generally in a longitudinal direction. In Bonellia the respiratory structures open into the latter chamber. Two kinds of excretory organs occur in some opening into the rectum, and in others into the alimentary cavity anteriorly. The protrusible proboscis is often armed with chitinous processes. The mouth opens at the base of the proboscis in the EchiuridaB, but at its tip in the Sipuncu- Iida3, the latter also having short ciliated tentacles sur rounding the aperture. It is followed by a pharynx and much-convoluted alimentary canal lined with cilia. The anus is either terminal, or situated dorsally at a point near the anterior third of the body. ^ The walls of the alimentary canal are glandular, and there are also muscular fibres. The nervous system consists of a ventral cord giving off various branches, but showing no distinct ganglionic en largements, nor indication of a fusion of two cords. There is an cesophageal collar, but the cephalic ganglia do not seem to be always distinct. There are no organs of the special senses except those of touch, which is fairly de veloped, and in a few eye-specks, especially in young forms.

The A. Gephyrea are dioecious, and have structures homologous with the segmental organs of the other groups, in the form of a series of tubes or ca3ca. In the Sqnm- culi, according to Keferstein and Ehlers, there are two testicles, and the ova are developed in ovaries attached to the wall of the body, but they vary in situation in other families. The products fall into the perivisceral cavity. In some the young undergo certain metamorphoses (Actino- trocha-iorm), but in others the larval condition differs from the adult chiefly in the possession of ciliated zones.

The Gephyrea are widely distributed on the surface of the globe, generally in muddy regions, and some are common in empty univalves. They are all marine. They have been grouped in three families: (1.) Echiurida;, contain ing forms with bristles, such as the common spoon-worm (Echiurus vulgaris) and Bonellia; (2.) Sipuncididce, with a dorsal anus, e.g., Phascolosoma Bernliardi of the univalve shells; (3.) Priapulidce, with a terminal anus, e.g., Priapuhis caudatus. Sternasjiis has lately been removed to the Poly- chseta, and Phoronis has been included in the group as a tubicolar Gephyrean.

The Chaetognatha and the higher Turbellaria approach

the Annelida proper very closely, though from different points of view, and may be regarded as intermediate between them and the Nematodes, Tematodes, and Cestodes. The Nemerteans (the highest group of the Turbellaria) especially come near the Annelida, notwith standing the condition of the nervous system. They have cilia externally, and a cutis which secretes similar hyaline

tubes to those of many Annelida. The muscular system ia