Echidna (ē-kid´na), a genus of Australian toothless mammals, in size and general appearance resembling a large hedgehog, excepting that the spines are longer and the muzzle is protracted and slender, with a small aperture at the extremity for the protraction of a long flexible tongue. The habits of the Echidna are nocturnal; it burrows, having short strong legs with five toes, and feeds on insects, which it catches by protruding its long sticky tongue. It is nearly allied to the Ornithorhynchus, the two forming a peculiar class of animals, having in their structure some peculiarities at once of mammals, birds, and reptiles. In 1884 it was found that, as Geoffrey St. Hilaire had suspected, the Echidna, the closely related Proechidna of New Guinea, and the Ornithorhynchus constitute the lowest sub-class of mammals, the Prototheria or Monotremata, which present many reptilian characters. They possess a cloaca into which the intestine and urinogenital ducts open and are oviparous. During the breeding season a temporary pouch is formed, and into this the milk-glands open, but there are no teats. The egg when laid is put into the pouch by the mother, and is there hatched in a very immature state. It feeds by licking up the milk in the pouch. Later on, the mother digs a burrow, where she leaves the young at night, returning during the day to suckle it. One species (E. hystrix), from its appearance is popularly known as the porcupine ant-eater.
Echinocactus (e-kī´-), a genus of cactaceous plants inhabiting Mexico and South America, and remarkable for their peculiar forms, being globular, oblong, or cylindrical, and without leaves, fluted and ribbed, with stiff spines clustered on woolly cushions. Some of them are very bulky. The flowers are large and showy. See Cactus.
Echinococcus, the very large compound cyst which forms the bladder-worm stage in the life-history of a small tapeworm (Tænia echinococcus) living in the intestine of the dog. The cysts are found in various abdominal organs of herbivorous animals, and sometimes infest human beings, especially in Iceland.
Echinodermata (e-kī-nō-dėr´ma-ta), a phylum or sub-kingdom of invertebrate animals characterized by having a tough integument in which lime is deposited in scattered plates (sea-cucumber), flexibly articulated plates (star-fishes), or so as to form a rigid test or shell like that of the sea-urchin; and by the radial arrangement of many of the parts of the adult. Movable spines are commonly present. There is a system of tubes into which seawater is admitted (ambulacral system), and commonly tube-feet, that are put into use by being distended with fluid. Some of them, as the encrinites or sea-lilies, are permanently fixed by a stalk when adult. Their development is accompanied by metamorphosis, and the embryo shows a distinctly bilateral aspect, though the radiate arrangement prevails in the adult. By some they are classed with the Scolecida in the sub-kingdom Annuloida. The phylum is divided into nine classes: Asteroidea (star-fishes); Ophiuroidea (brittle-stars), Echinoidea (sea-urchins), Holothuroidea (sea-cucumbers), Crinoidea (sea-lilies, feather-stars, the latter free-moving), Thecoidea or Edrioasteroidea (extinct, stalkless but fixed), Carpoidea (extinct, stalked), Cystoidea (extinct, stalked), Blastoidea (extinct, stalked). All are marine.
Echinomys, or Spiny Rat, a genus of South American rodent mammals distinguished by the presence of spines among the coarse fur. The long tail is covered by scales and hair intermixed, the ears are large, and all the extremities possess five digits.
Echinus (e-kī´nus), Sea-urchin, or Sea-egg, a genus of marine animals, the type of an order (Echinoidea) of the phylum Echinodermata (see above). In this type the body is spheroidal and invested in a test or shell composed of regularly arranged plates closely united together. It is covered with movable spines articulated by ball-and-socket joints. The mouth is situated in the centre of the under surface, and there is a complicated masticatory apparatus (Aristotle's lantern) consisting of five chisel-ended teeth
supported by an elaborate framework. The anus is similarly placed on the upper side, and is surrounded by a circlet of ten plates (apical disc), one of which bears a furrowed tubercle (madreporite) perforated by small holes through which water enters the water-vascular system. Locomotion is effected by meridional rows of tube-feet, aided by the spines. E. esculentus and some other species are edible. See Sea-urchin.
Echinus (e-kī´nus), in architecture, the ovolo or quarter-round convex moulding, seen in capitals of the Doric order. It is especially frequently found carved with the egg-and-dart ornament.
Echo (ek´ō), the repetition of a sound caused by the reflection of sound-waves from some surface, as the wall of a building. The echo may, however, be very distinct when the reflecting surface is very irregular, and it is probable that the resonance of the obstacles and the masses of air which they enclose contribute in producing the echo. The waves of sound on meeting the surface are turned back in their course according to the same laws that hold for reflection of light. In order that the echo may return to the place from which the sound proceeds, the reflection must be direct, and not at an angle to the line of transmission, otherwise the echo may be heard by others but not by the transmitter of the sound. This may be effected either by a reflecting surface at right angles to the line of transmission, or by several reflecting surfaces which in the end bring the sound back to the point of issue. Sound travels about 1125 feet in a second; consequently, an observer standing at half that distance from the reflecting object would hear the echo a second later than the sound. Such an echo would repeat as many words and syllables as could be heard in a second. As the distance decreases, the echo repeats fewer syllables till it becomes mono-syllabic. The most practised ear cannot distinguish in a second more than from nine to twelve successive sounds, so that a distance of not less than 60 feet is needed to enable an average ear to distinguish between the echo and the original sounds. At a near distance the echo only clouds the original sounds, and this often interferes with the hearing in churches and other large buildings. Woods, rocks, and mountains produce natural echoes in every variety, for which particular localities have become famous.