HOLOPTTCHIIIS (holes, all, and plyche, wrinkle), a remarkable genus of fossil ganoid fishes, so named from the wrinkled appearance of the enameled scales. They were of large size, some species probably the length of 12 feet. The small head was covered with large tuberculated plates, like those of the crocodile, and the body was completely encased in large scales, more like those of a reptile than a fish. Some scales have been found measuring 3 in. in length by 24 in breadth, and a full eighth of an inch in thickness. They were composed internally of porous bone, in numerous layers, arranged alternately at right angles to each other, and the outside was covereu with a bright glossy corrugated enamel. The spines of the fins were large and hollow; the bones were partially ossified; the center remained in its original cartilaginous condition, and consequently appears hollow in the fossil. The jaws were covered with hard enamel instead of skin, and were furnished with a double row of teeth; the outer row, placed along the edge of the mouth, were small and thickly set; the inner range were widely set, and very large, at least twenty times the bulk of the others. A specimen was obtained at Clashannie, on the firth of Tay, by Mr. Noble; it now forms part of the British museum collection. It is a foot across by 24 ft. long without the tail, which is wanting. It is nearly perfect, lying on its back, with the scales and the ventral fins in their original position.
The genus. is peculiar to the old red sandstone and carboniferous measures; eight species being found in the former, and nine in the latter. The name holoptychins i. now generally confined to the fossils of the old red sandstone, and that of rhizodus. which Owen applied to the teeth remains before their connection with the fish Nvas known, has been given to the holoptychians of the coal measures, which have the outer row of teeth more robust and obtuse, and the inner set longer, sharper, and more slender than in the older species.
HOLOTHIrRIA, a genus of echinodermata (q.v.), the former limits of which are now those of a family, holothurida, divided into numerous genera. SEA-SLUG and SP:A-CUCUM BER are popular names of some of the animals of this family. The holothuridee have not the covering of calcareous plates characteristic of the more typical cchinodermata, but a soft leathery muscular integument, - very irritable, and capable of great distention and contraction. Some of them are almost globose, some so much elongated as to be almost
worm-like; but the same individual is often capable of extending itself to several times the length which it has in a state of repose. In locomotion, the body is extended and contracted as by the annelides, but the principal organs of locomotion, as in star-fishes and sea-urchins, are suckers or ambulacra (q.v.), of which there are usually five double rows, whilst sometimes they are distributed over the whole surface of the body; but some of the species have the suckers developed only on a disk, and the body then pre sents an upper and an under surface. The radiate structure is most apparent in the mouth, which is surrounded with tentacles, in number always a multiple of five, exhib iting great variety of beautiful forms, and capable of being completely retracted. Little is known of the food of the holothurida, which, however, probably consists of small marine animals. Within the opening of the mouth there is a circle of teeth. There is no proper stomach. The intestine is often very complicated. The respiratory organs are near the anus, and consist of branching tubes. The organs of both sexes are found in each individual. The young pass through several stages or transformations, in which they are very unlike their parents; in their first stage, after leaving the egg. they swim vigorously by means of membranous expansions of the body. 1 he are capable of the most extraordinary reproduction of parts, even of the most important organs. They are found in all seas, but particularly- abound in the Red sea, and between the s. of Asia and Australia. The largest European species, II. (cueumaria) fron dosa, occasionally found in the British seas, is about a foot in length. and capable of extending itself to 3 feet. Most of the British species are small. and they are not of a pleasing appearance as they usually come under observation, -although the expanded tentacles give their beauty in their proper abodes. .But Many of the tropical species exhibit splendid colors, and arc among the creatures which make the bottom of the sea particularly among coral reefs and islands, gay and lovely as a garden.
The Melte-de-771er (q.v.), or trepang, so much esteemed as a delicacy by the Chinese belongs to this family.