RHIZ′OCEPH′ALA (Neo-Lat. nom. pl., Gk. ῥίζοκέφαλος, rhizokephalos, having the flower growing immediately from the root, from ῥίζα, rhiza, root + κεφαλή, kephalē, head). An order of most degraded cirripeds, the root-barnacles, especially of the genera Sacculina and Peltogaster. The few forms known are parasitic on the abdomens of various crabs.
RHIZ′ODON′TIDÆ (Neo-Lat. nom. pl., from Gk. ῥίζα, rhiza, root + ὁδούς, odous, tooth). A family of carnivorous Paleozoic fishes of the crossopterygian suborder Rhipidistia, in which the median fins are always more or less subdivided by a process of concentration analogous to that prevalent among dipnoans. The notochord was persistent, and the jaws bore many peculiar teeth. These teeth were highly complicated in the Holoptychidæ, less so in the Rhizodontidæ, with a larger central cavity. A well-known example, found in an unusually good state of preservation, is Eusthenopteron Foordi, herewith figured, which abounds in the Devonian rocks of Quebec, and whose pectoral fin has only one stout basal cartilage—an ‘archipterygium.’ All were small fishes. Consult Woodward, Vertebrate Paleontology (Cambridge, 1898).
EUSTHENOPTERON FOORDI.
RHIZOIDS (from Gk. ῥίζωδής, rhizōdēs, root-like). Colorless, hair-like structures developed by liverworts, certain algæ, mosses, and fungi (toadstools and molds). Their function is to anchor the plant in favorable situations and in some cases perhaps to absorb water. The ferns have rhizoids only in connection with the very small prostrate sexual plant (gametophyte, q.v.).
RHIZOP′ODA (Neo-Lat. nom. pl., from Gk. ῥίζα, rhiza, root + ποῦς, pous, foot). The first and lowest class of Protozoa (q.v.); a large assemblage of varying forms agreeing in the possession of projections of the body-protoplasm called ‘pseudopodia.’ These pseudopodia are used as organs of locomotion and also for obtaining food. They may be of very irregular and constantly changing shape or comparatively rigid, independent of each other, or forming a very complex network. The protoplasm or ‘sarcode’ of a rhizopod consists of an outer layer called ‘ectosarc,’ which is thin, transparent, and homogeneous, and an inner portion called ‘endosarc,’ which is granular and more opaque. Most of the numerous species are spherical with radiating pseudopodia, but the lowest forms have no constant shape. Most rhizopods are provided with some sort of shell, but the lowest forms have no such covering. The simplest shells are those made up of particles of dirt or foreign material of some sort, united together by some secretion of the ectosarc; in other cases the shell is formed of a horny material called ‘acanthin’ or of carbonate of lime or silica, often in a very remarkable and elaborate pattern. Generally rhizopods are freely moving animals, but some are attached in adult life by stalks. The individuals are usually distinct, but colonial rhizopods are known, and such colonies are sometimes half an inch across.
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1. A radiolarian, showing the interior; 2, cross-section of the same through the processes. |
The only internal organs of the Rhizopoda are the ‘vacuoles,’ those which contain more or less digested food and those which contain the waste matter or excreta of the body. The latter are the larger and more conspicuous, and, owing to the sudden collapse when the excreta are thrown out of the body, are known as ‘contractile’ vacuoles. Reproduction takes place by simple budding or fission, the two processes differing only in the relative size of the resulting individuals. The formation of spores, however, occurs in many cases, especially after a resting period. Such resting periods occur during unfavorable conditions such as prolonged cold or drought, or when an unusual amount of food has been taken. In most cases during such a resting period the rhizopod surrounds itself with a firm, impervious coat, called a ‘cyst.’ When the unfavorable conditions cease the sarcode divides up into several minute spores, each of which on the dissolution of the cyst becomes a new individual. The growth of these individuals is rapid when food is abundant. For further information, see Bütschli's Protozoa in Bronn's Klassen und Ordnungen des Thierreichs (Leipzig, 1887); concerning fresh-water forms, see Leidy, Fresh Water Rhizopods of North America (Washington, 1879).
RHODE ISLAND. A North Atlantic State of the United States, belonging to the New England group, and situated between latitudes 41° 18′ and 42° 3′ N. and between longitudes 71° 8′ and 71° 53′ W. It is bounded on the north and east by Massachusetts, on the south by the Atlantic Ocean, and on the west by Connecticut. Rhode Island is the smallest in area of the United States. It has an extreme length from north to south of 48 miles, and an extreme width from east to west of 36 miles. Its area is 1250 square miles, of which 1053 square miles, or 673,920 acres, are land surface.
Topography. The greater part of the State belongs to the eastern Appalachian belt known in the Southern States as the Piedmont Plain. It is rough and hilly, though the general elevation is not great, being less than 600 feet throughout the State, with the exception of a few monadnocks. The highest point is Durfee Hill, near Gloucester, in the northwestern part; its altitude is 805 feet. The western part of the coast-line is bordered by lagoons and marshes, while the eastern part of the State is separated from the main portion by Narragansett Bay, a large, irregular, and branching inlet extending 28 miles inland, with a breadth of from 12 to 3 miles. It incloses a number of islands, the
