624
ECHIUROIDEA
many cases only a portion is transformed into the adult Echinoderm (where care of the brood has secondarily arisen, this larva is not developed). All living, and most extinct, Echinoderms show the following features, almost certainly due to an ancestral Pelmatozoic stage :—An incomplete radial symmetry, of which five is usually the dominant number, is superimposed on the secondary bilateralism, owing to the outgrowth from the mouth region of one unpaired and two paired ciliated grooves; these have a floor of nervous epithelium, and are accompanied by subjacent radial canals from the water-ring, giving off lateral podia and thus forming ambulacra, and by a perihsemal system of canals apparently growing out from ccelomic cavities. All living Echinoderms have a lacunar, haemal system of diverse origin; this, the ambulacral system, and the coelomic cavities, contain a fluid holding albumen in solution and carrying numerous amcebocytes, which are developed in special lymph-glands and are capable of wandering through all tissues. The Echinoderms may be divided into seven classes, whose probable relations are thus indicated:— ' Cy stidea Edrioasteroidea
short distance behind the mouth, and are moved by special muscles; they are of use in helping the animal to move
Holothurioidea'
PELMATOZOABlastoidea Crinoidea
-ELETJTHEItOZOA Asteroidea (and Ophiuroidea) Echinoidea Authorities.—In addition to the works referred to in the opening paragraph, the following deal with the general subject Bather, Gregory, and Goodrich. “ Echinoderma, ” in Lankester’s Treatise on Zoology. London, 1900.—Bell. Catalogue of the British Echinoderms in the British Museum. London, 1892.—P. H. Carpenter. “ Notes on Echinoderm Morphology,” Quart. Journ. Micr. Sci., 1878-1887.—Lang. Text-Book of Comparative Anatomy, transh, part ii. London, 1896.—Ludwig and Hamann. “ Echinodermen,” in Bronn’s Klassen und Ordnungen des Thierreichs. Leipzig, 1889—in progress.—Neumayr. Die, Stdmme des Thierreiches. Wien, 1889.—P. B. and C. F. Sarasin. “Ueber die Anatomic der Echinothuriden und die Phylogenie der Echinodermen,” Ergebnisse naturw. Forsch. avf Ceylon, Bd. i. Heft 3. Wiesbaden, 1888.—Semon. “DieHomologien innerhalb des Echinodermenstammes,” Morph. Jahrh., 1889. —Sladen. “Homologies of the Primary Larval Plates in the Test of Brachiate Echinoderms,” Quart. Journ. Micr. Sci., 1884.— Zittel. Handbuch der . . . Palceozoologie, i. pp. 308-560. Miinchen, 1879; also Grundzilge, translated and revised by Eastman as Text-Book of Palaeontology. Hew York and London, 1899. 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.) Echiuroidea.- — The Echiuroidea form a small group of marine animals which show in their larval lifehistory 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 group, which by the proposed further separation of the Priapuloidea on account of their unique renal and reproductive organs, has practically ceased to exist. 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 Bondlia 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
Fig. 1.—A, Bonellia viridis, Eol., 9 > A B. fuliginosa. Both natural size, a, grooved proboscis; b, mouth; c, ventral hooks; d, anus. 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 nighttime, 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 as bait. 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 ccelomic 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 sensecells 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
