Page:EB1911 - Volume 27.djvu/257

Rh into a number of independent suckers set on a disc or “cotylophore.” Eye-spots are general and the nervous system maintains a primitive diffused condition. The excretory system opens to the exterior by a pair of dorsal pores at the level of the pharynx. The eggs are comparatively few, and development is direct, the embryo after reaching its host remaining attached to it for life.

All the members of this order are parasitic on aquatic vertebrates and in rare cases derive their food from a vertebrate host indirectly by means of another invertebrate parasite (e.g. Udonella occurs on parasitic Crustacea). They are transparent leaf-like organisms and may often be found attached to the skin, mouth, nostrils or gills of fish; on the skin and bladder of Amphibia; and on those of certain Reptilia. Polystomum integerrimum (fig. 5) occurs commonly in the “bladder” of frogs and toads; Diplozoon on the skin of the minnow; Gyrodactylus (figs. 5, 6) on the gills of various fresh-water fish; and a large number of genera occur on the skin, cloaca and gills of Elasmobranchs and other marine fish. They ingest the mucus and, to some extent, the blood of their host by the aid of a sucking pharynx through which the food passes into the bifurcated alimentary sac and its branched caeca.

The life-history of this order offers many points of interest. The eggs are stalked and provided with chitinoid often operculate shell. Each shell contains a single ovum and a mass of yolk-cells. In most cases the eggs are attached to the host, but in Polystomum the eggs are laid in water. The egg of Gyrodactylus develops in the body of the parent.

The further history of the animal is only known in a few cases. Polystomum hatches out six weeks after ovi-position as a minute (.3 mm. long) larva capable of swimming freely for a short time by the aid of five girdles of ciliated cells. If in the course of the first twenty-four hours this larva meet with a tadpole it attaches itself at once and undergoes further development. If unsuccessful it dies. In the former case the larva creeps along the tadpole until it reaches the branchial opening into which it darts, fixes its sucker, and then throws off its cilia. Its further development takes place partly in the branchial chamber and partly in the bladder, which it reaches by travelling the whole length of the alimentary canal. In the former position the suckers are developed and growth proceeds for 8 to 10 weeks until the metamorphosis of its host. In the bladder it remains for three years before attaining maturity. Sometimes the Polystomum-larva attaches itself to a young tadpole, and in that case grows so rapidly as to become mature in five weeks. These Polystomum deposit their eggs in the branchial chamber and die at the metamorphosis of their host. They differ structurally

from the normal form in being capable of self-fertilization only, and in the shape and details of their spermatozoa.

The life-history of Diplozoon (fig. 5) is remarkable in that two larvae (the so-called Diporpae) unite and fuse permanently into an X-shaped organism. Unless this occurs, the development of the larvae is soon arrested. The ciliated stage is only capable of free life for five or six hours, and if at the end of that time it has not encountered and attached itself to a minnow, it dies. If successful, the larva throws off its cilia and develops a dorsal papilla, a median ventral sucker and an additional pair of lateral suckers. Then the Diporpa stage is attained. This stage is capable of isolated existence for two or three months but remains immature. Should it, however, encounter another Diporpa, the mid-ventral sucker of either is applied to the dorsal papilla of the other, and complete fusion takes place across the junction. The compound organism now develops two sets of inter-connected genitalia and becomes a Diplozoon.

Gyrodactylus produces only one large egg at a time and this develops in situ into an embryo: but within this embryo another appears before the first leaves the parent. This anomalous phenomenon is still obscure, for we do not yet know whether the second embryo is developed sexually or asexually from the first. Von Linstow has indeed suggested that Gyrodactylus is a larval form capable of reproduction by an asexual method.

Order 2. Aspidocotylea.—Endoparasitic Trematodes provided with a large ventral sucker which is almost co-extensive with the lower surface of the body and is divided into rectangular compartments. The alimentary sac is simple and devoid of caeca. The development is direct.