of Palaeontology (New York and London, 1899). The larger treatises here mentioned contain very full bibliographies, and a complete analytical index to the annual literature of the Echinoderma has for many years been published in the Zoological Record (London). (F. A. B.)
ECHINUS (Gr. for “hedge-hog” or “sea-urchin”), in architecture, the convex moulding which supports the abacus of the Doric column. The term is sometimes given to the ovolo of the Ionic capital, especially when curved with the egg-and-tongue enrichment. The origin of this use of the word in architecture, which comes down from ancient times, is uncertain.
ECHIUROIDEA (Gr. ἔχις, adder, and οὐρά, tail), the zoological name for a small group of marine animals which show in their larval life-history a certain degree of segmentation, and are therefore grouped by some authorities as Annelids. Formerly, together with the Sipunculoidea and Priapuloidea, they made up the class Gephyrea, but on the ground that they retain in the adult a large preoral lobe (the proboscis), that they have anal vesicles, that their anus is terminal, that setae are found, and finally that they are segmented in the larval stage, they have been removed from the class, which by the proposed further separation of the Priapuloidea on account of their unique renal and reproductive organs, has practically ceased to exist.
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| Fig. 1.—A, Bonellia viridis, Rol., ♀; B, B. fuliginosa. Both natural size. a, grooved proboscis; b, mouth; c, ventral hooks; d, anus. |
Echiuroids are animals of moderate size, varying roughly from one to six or seven centimetres in length, exclusive of the proboscis. This organ is capable of very considerable extension, and may attain a length in Bonellia viridis of about a metre and a half (fig. 1). It is grooved ventrally and ciliated. At its attachment to the body the groove sinks into the mouth. In Bonellia the proboscis is forked at its free end, but in the other genera it is short and unforked. The body is somewhat sausage-shaped, with the anus at the posterior extremity, surrounded in Echiurus by a single or double ring of setae. The skin is usually wrinkled, and in B. viridis, Thalassema lankesteri, Th. baronii, Hamingia arctica, and in the larva of many species, is of a lively green colour. A pair of curved bristles, formed in true setal sacs as in Chaetopoda, project from the body a short distance behind the mouth, and are moved by special muscles; they are of use in helping the animal to move slowly about, and they take a large share in the burrowing movements (C. B. Wilson, Biol. Bull., 1900), for some species tunnel in the mud and sand and form more or less permanent burrows, the walls of which are strengthened by mucus secreted from the skin. The openings of the burrows become silted up, leaving, however, a small aperture through which the proboscis is extruded. This organ carefully searches the neighbourhood for particles of food. When these are found the grooved proboscis folds its walls inwards, and the cilia pass the particles down the tube thus formed to the mouth. Echiuroids also move by extending the proboscis, which takes hold of some fixed object, and, then contracting, draws the body forwards. Recently it has been shown that Echiurus swims freely at night-time, using for locomotion both the proboscis and the contraction of the muscles of its body-wall. The motion is described as “gyratory,” and the anterior end is always carried foremost. Those species which do not burrow usually conceal themselves in crevices of the rocks or under stones, or at times in empty Mollusc or Echinid shells. They are occasionally used by fishermen for bait.
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| Fig. 2.—Female Bonellia viridis, Rol. Opened along the left side. | |
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a, Proboscis cut short. |
f, Ovary borne on ventral vessel |
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| Fig. 3.—Adult male, Bonellia viridis, Rol. The original was 1·5 mm. long. The nervous system is not shown. (After Selenka.) | |
| a, | Generative pore with spermatozoa coming out.
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| b, | Anterior blind end of intestine attached to the parenchymatous tissue by muscular strands.
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| c, | Green wandering cells containing chlorophyll.
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| d, | Parenchymatous connective tissue. |
| e, | Epidermis. |
| i, | Intestine. |
| j, | Vas deferens. |
| l, | Internal opening of vas deferens. |
| m, | The left anal vesicle. |
| n, | Spermatozoa in the body-cavity. |
Anatomy (fig. 2).—A thin cuticle covers the epidermis, which contains mucus-secreting glands. Beneath the epidermis is a layer of circular muscles, then a layer of longitudinal, and finally in some cases a layer of oblique muscle-fibres. The inner face of this muscular skin is lined by a layer of epithelium. The coelomic body-cavity is spacious. It does not extend into the proboscis, which is a solid organ traversed by the nervous and vascular rings, but otherwise largely built up of muscle fibres and connective tissue. Many sense-cells lie in the epidermis. The ciliated ventral groove of the proboscis leads at its base into the simple mouth, which gives access to the thin-walled alimentary canal. This is longer than the body, and to tuck it away it is looped from side to side. The loops are supported by strands of connective tissue, which in some species are united so as to form a dorsal mesentery, whilst traces of a ventral mesentery are met with anteriorly and posteriorly (H. L. Jameson, Zool. Jahrb. Anat., 1899). The alimentary canal is divisible into fore-gut, mid-gut and hind-gut, and the first-named can be further divided into pharynx, oesophagus, gizzard and crop, mainly on histological grounds. The mid-gut is characterized by the presence of a ciliated groove, from which arises the collateral intestine or siphon, a second tube which rejoins the alimentary canal lower down. Similar collateral intestines are familiar in the Echinids and certain Polychaets (Capitellidae). The rectum receives the openings of a pair of very characteristic organs, the anal vesicles. Each consists of a branching tube, the tips of whose twigs terminate in minute ciliated funnels.
