V. Annelida Gephyrea
Annelida Gephyrea: The Body. Circulatory System. Digestive System. Nervous System. Reproductive System. Distribution. A Note on Classification.
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 segmented. 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 rugae, and many pores. Beneath is a hypoderm containing certain glandular organs or sacs, and, in some, bodies like tricho cysts. Bristles occur in Echiuris and Bonellia. 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 and of the body to the other. The circulatory system shows a dorsal and ventral vessel, both in the Sipunculidae communicating with a circular vessel (ciliated internally) surrounding the oesophagus, and sending prolongations into the ciliated tentacles. The latter contains a corpusculated 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 longitudinal 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 Echiuridae, but at its tip in the Sipunculidae, the latter also having short ciliated tentacles surrounding 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 enlargements, nor indication of a fusion of two cords. There is an oesophageal 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 developed, 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 caeca. In the Sipunculi, 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 (Actinotrocha-form), 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 threefamilies: (1.) echioridoe, containing forms with bristles such as the common spoon-worm (Echiurus vulgaris) and Bonellia; (2.) Sipunculidoe, with a dorsal anus, e.g., Phascolosoma Bernhardi of the univalve shells; (3.) Priapulidoe, with a terminal anus, e.g., Priapulus caudatus. Sternaspis has lately been removed to the Polychaeta, and Phoronis has been included in the group as a tubicolar Gephyrean.
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