The movements and activity of the redia cause it to burst the wall
of the sporocyst. It escapes into the adjacent tissue and there gives
rise either to one or more generations of rediae or at once to a new
type of organism—the cercaria. What determines the origin of
the cercaria rather than a new generation of rediae is unknown.
It originates from germ-balls by a differentiation similar in general
to that already described, though profoundly different in detail.
The cercaria is just visible to the naked eye and has an oval or
discoidal body and usually a long tail of variable form. The tail
may be a simple hollow muscular process or provided with stiff
bristles set in transverse rows, or divided into two equally long
processes, or finally it may form a large vesicular structure. The body
contains in miniature all the organs of the adult fluke, including
the gonads and in addition “eye-spots,” a stylet, rod-cells and
cystogenous cells. The latter structures are only employed for an
interval before the final host is entered.
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| Fig. 9. |
|
A, Schistostomum (Bilharzia) haematobium, the thin female in the
gynaecophoric canal of the stouter male. ( B, Distomum macrostomum, showing the digestive and the greater part of the genital apparatus with the cirrus protruded. C, Snail (Succinea), the tentacles deformed by Leucochloridium. (Natural size.) D, Leucochloridium removed from the tentacle. (Natural size; after Zeller.) E, Bucephalus polymorphus. (Highly magnified; after Ziegler.) F, Portion of a sporocyst containing Bucephali in process of development. (× about 50; after Lacaze-Duthiers.) |
The number of cercariae produced by the pullulating rediae in a single water-snail is immense, and as they are emitted at a given period or a few successive periods, the snail at these times appears enclosed in a cloud of whitish flocculent matter. The cercaria swims freely for a time and either encysts directly on grass or weeds or it enters a second host which may be another mollusc, an insect, crustacean or fish, and then encysts. In this process it is aided by the stylet with which it actively bores its way, throws off its tail and then, surrounding itself with the secretion of its cystogenous cells, comes to rest. The further development of the cercaria is dependent on the weed or animal in which it lies being eaten by the final host which is usually a predaceous fish or one of the higher vertebrates. When that occurs, the cyst is dissolved and the minute fluke works its way down the alimentary canal into some part of which it inserts its suckers and commences to feed on the blood of its host. Occasionally the fluke migrates into the blood vessels and may reach the lungs, kidneys, urethra and bladder. In the course of a few months it attains full size and maturity and probably in most cases dies in the course of a year after having given rise to another generation of larvae.
A few special cases of this general description of the life-history may be mentioned. The liver-fluke (Distomum hepaticum) passes through its larval stages in the water snail Limnaea truncatula in Europe; in L. oahuensis in the Hawaiian Islands; in L. viator in South America and in L. humilis in North America: and is eaten by sheep during its encysted stage attached to herbage. Distomum macrostomum, which occurs in various birds, produces a very curious sporocyst in the body of the snail Succinea putris. This sporocyst assumes a branched structure and penetrates into the tentacles of the snail (fig. 9, c, d). In this situation it becomes much swollen and banded with colours, and produces a large number of ecaudate cercariae. The attention of birds is speedily attracted to the snail by this appearance and by the peculiar movements which the worm executes, and the passage of the parasite into its final host is advantageously effected. In many cases it appears that only the brilliantly coloured tentacle is pecked off by the bird, and as the snail can easily regenerate a new one, this in turn becomes infected by a fresh branch of the sporocyst ramifying through the snail and thus a new supply of larvae is speedily provided (Heckert).
The life-history of Schistostomum haematobium is still unknown, but the difficulty in obtaining developmental stages in any of the numerous intermediate hosts that have been tried suggests that the ciliated larvae may develop directly in man and either gain access to him by the use of impure water for drinking or may perforate his skin when bathing. Experiments on monkeys have, however, given negative results.
The life-history of the Holostomidae differs from that of the Distomidae in an important regard. These Trematodes live chiefly in the intestine of aquatic birds or reptiles. The ciliated larva escapes from the egg into the water and enters an intermediate host (leech, mollusc, arthropod, batrachian or fish) where it undergoes a metamorphosis into a second stage in which most of the adult organs are present. In this condition they remain encysted as immature flukes until eaten by their final host.
The cycle of development taken by the Malacocotylea has been generally regarded as an alternation of one or more asexual generations with a sexual one. The question, however, is complicated by the uncertain nature of the germ-cells in the sporocysts and rediae. Some authors looking upon these as parthenogenetic ova regard the developmental cycle as one composed of an alternation of parthenogenetic and of sexual generations. Others again consider that the whole cycle is a metamorphosis which, beginning in the Heterocotylea as a direct development, has become complicated in the Holostomidae by a larval history, and finally in the Malacocotylea has acquired additional complexity by the intercalation of two larval forms, and is thus spread over several generations.
Literature.—R. Leuckart, Die Parasiten des Menschen (1889-1894), vol. ii.; M. Braun, “Trematodes,” Klassen u. Ordnungen des Tierreichs (1889-1893), vol. iv. (Monograph), and The Animal Parasites of Man (London, 1906); W. B. Benham in Lankester's Treatise on Zoology (1901), pt. iv.; A. Heckert, “Untersuchungen über die Entwicklung und Lebensgeschichte des Distomum macrostomum,” Bibliotheca zoologica, Heft 4 (Cassel, 1889); J. T. Cunningham, “On Stichocotyle nephropsis,” Trans. Roy. Soc. Edin. (1887), vol. xxxii.; A. Looss, “Die Distomen unserer Fische und Frosche,” Bibliotheca zoologica (1894), Heft 16; H. L. Jameson, “Pearl-formation,” Proc. Zool. Soc. p. 140 (London, 1902); A. E. Shipley and J. Hornell, “Parasites of the Pearl Oyster,” Report on the Pearl Oyster Fisheries of the Gulf of Manaar, The Royal Society (1904), pt. ii. pp. 90-98; F. Sommer, “Anatomy of Liver-fluke,” Zeit. f. wiss. Zoologie (1880), vol. xxxiv.; Thomas, “Development of Liver-fluke,” Quart. Journ. Mic. Sci. (1883), vol. xxiii.; Jägerskiold, Fauna arctica.
TREMOLITE, a member of the amphibole group of rock-forming minerals (see Amphibole). It is a calcium and magnesium meta silicate, CaMg3(SiO3)4, crystallizing in the monoclinic system with an angle of 55° 49' between the perfect prismatic cleavages. It occurs sometimes as distinct crystals, but more usually as long bladed and fibrous forms. The colour is white or grey, but when iron is present it is green, then forming a passage to actinolite. The hardness is 5½ and the specific gravity 3.0. Tremolite is a characteristic mineral of crystalline limestones, especially dolomitic limestones, but also occurs as an alteration-product of olivine in basic igneous rocks. Typical specimens have long been known from the white crystalline dolomite of Campolongo in the St Gotthard region, Switzerland, near to which is the Tremola Valley, after which the mineral was named in 1796. Fine crystals are found in crystalline limestone at Gouverneur, Pierrepont and other places in New York, and at several localities in Sweden. (L. J. S.)
TRENCH, RICHARD CHENEVIX (1807-1886), Anglican archbishop and poet, was born at Dublin on the 9th of September 1807. He went to school at Harrow, and graduated at Trinity College, Cambridge, in 1829. In 1830 he visited Spain. While incumbent of Curdridge Chapel near Bishops Waltham in Hampshire, he published (1835) The Story of Justin Martyr and Other
