Page:EB1911 - Volume 27.djvu/258

Rh These Trematodes occur in the alimentary canal and adjacent organs of Mollusca, the gall-bladder of Chimaera, and the intestine of Chelonia and of certain fish. Aspidogaster conchicola is a form not uncommon in Anodon, Unio and certain fresh-water Gastropods. When young it is found in the intestine, but becomes mature in “Keber's organ” and the pericardium. An allied form (A. margaritiferae) occurs in the pericardium of the Ceylon pearl-oyster (9).

This order differs in several points from the preceding one. The excretory system is highly developed and opens at the posterior extremity by a paired muscular bladder. The testis is a single compact organ. From the oviduct a long duct full of yolk passes backwards almost to the hinder end of the body and ends blindly in a globular dilatation just below the skin. This structure is regarded as the homologue of a canal (Laurer's canal) which in the Heterocotylea opens into the intestine and so gets rid of the excess of yolk. The life-history of the order is almost unknown, but at the time of hatching the young Aspidocotylean has an oral sucker at the anterior extremity and an equally simple post-oral one at the other, thus resembling the members of the next order. Subsequently the body grows backwards and the ventral sucker comes to occupy a relatively more anterior position. Concomitantly its cavity is sub-divided by transverse ridges into a single row and later on into paired rows of compartments. A curious form (Stichocotyle) described in an immature condition by Cunningham from the lobster and Norway lobster probably belongs to this order.

Order 3. Malacocotylea (Distomae, Leuck: Digenea v. Ben.). Endoparasitic Trematodes with a variable adhesive apparatus. The oral sucker may alone be present (Monostomidae), more usually a second is developed on the under surface, but may be mid-ventral (Distomidae) or terminal. It is posterior (Amphistomidae), or anterior (Gasterostomidae). In addition to these suckers the sides of the anterior region may become infolded and give rise to an accessory adhesive organ (Holostomidae). In all these families spines and glandular papillae may be super-added. The intestinal sac has become bifid and is usually devoid of branches. The excretory system is highly developed; the larger collecting ducts are elaborately looped and open posteriorly by a single terminal aperture. A canal (Laurer's canal) leads from the oviduct or yolk-duct to the dorsal surface. The development is indirect. From the egg a larva arises. This enters a temporary host. Here it gives rise by a peculiar process to numerous individuals of a second larval form, and these usually produce a third form from which the minute immature Trematode is developed. In this manner a single egg may give rise to a large number of sexual individuals. The larvae usually live in Molluscs, the mature worm in vertebrates, and the immature but metamorphosed Trematode in either host and also in pelagic and littoral marine and fresh-water invertebrates.

The Malacocotylea occur in all classes of vertebrates. They are usually found in the alimentary canal or its appendages but occasionally work their way into the serous cavities, nervous system and blood vessels. Fourteen species belonging to five genera have been found in man, but only one [Schistostomum (Bilharzia) haematobium] is of serious medical importance, the others being rare and occasioned by want of cleanliness and close association with infected domestic animals. Domestic animals suffer periodically to a much greater extent. The liver-fluke (Distomum hepaticum) unlike most Trematodes flourishes in a wide range of hosts and infects man, horse, deer, oxen, sheep, pig, rabbit and kangaroo. Sheep, however, suffer most from this parasite and from the allied D. magnum. The former fluke is found in Europe, North Africa, Abyssinia, North Asia, South America, Australia and the Hawaiian Islands; the latter in the United States. Wet summers are followed by an acute outbreak of liver-rot amongst sheep and this, together with the effects of other diseases that accompany wet seasons, cause the death of vast numbers of sheep, the numbers from both sources being estimated in bad years at from 1½ to 3 millions in England alone. The anatomy of Distomum hepaticum is fully described in many accessible memoirs [Sommer (10), Marshall and Hurst, Braun (3)]. It has been shown that this parasite feeds upon the blood, not the bile of its host, though it occurs mainly in the bile ducts.

The life-histories of the Malacocotylea form the most interesting feature of the order. The majority of species are hermaphrodite and many are capable of self-impregnation. In these, the male organs ripen before the ova and spermatozoa may pass into the uterus before the external pore is formed (Looss). A few species, however, are bisexual, e.g. Schistostomum (Bilharzia) haematobium in which the male is larger than the female and encloses the latter in a ventral canal; Koellikeria filicolle Rud (Distomum okenii, Köll) which also occurs in pairs, a large female and a small male being found together encysted in the branchial chamber of Brama raji:

and Didymozoon thynni (Monostomum bipartitum) which occurs in pairs fused for the greater part of their length and only free anteriorly; the larger individual is the female.

The egg consists of a fertilized ovum and a mass of yolk-cells. Segmentation takes place during its passage down the uterus. The result of this process is a minute ovoid embryo consisting of a solid mass of cells surrounded by a follicle of flattened yolk-cells. The central mass soon becomes differentiated into an outer epidermal and a dermal layer of flat-cells. Some of the central cells remain in clumps as “germ-balls,” others form a mesenchyma in which “flame-cells” arise; others again give rise to muscles; and at the thicker end of the body, rudiments of the brain and digestive system are observable. A pair of “eye-spots” develops immediately over the brain. If the egg with its contained embryo falls into water with the faeces of the host the larva hatches out and swims freely for a time. In dry localities or in the absence of the intermediate host (usually a mollusc) this larva soon dies. If, however, it encounters the host the larva bores its way in, and attacks the liver, mouth or gonad in which it comes to rest. In all Malacocotylea except the Holostomidae the ensuing change is a degenerative one. The cilia are lost, the eye-spots disappear, the digestive sac vanishes and the larva becomes a sac or “sporocyst” full of germ-cells. The origin of these cells is a moot point. According to some writers (Leuckart) they are derived from undifferentiated blastomeres, other authorities (Thomas, Biehringer, Heckert) trace them to the parietal cells of the larva. These cells aggregated in masses become the bodies of another generation of larvae within the sporocyst. By a series of changes similar to those by which the primary larva arose from a segmented egg, so do these secondary larvae or “rediae” arise from the germ-cells or germ-balls within the sporocyst. The structure of a redia, however, is an advance on that of its parent. Though not possessing eyes or cilia, it has a pharynx and short straight digestive sac; and its mesenchymatous cavities are filled with germ-balls in various stages of development.