(387). "We have thus followed the Distoma to its third stage of ascent, and, as no more stages in the generations of these animals have been detected, are not in a condition to trace the origin of the Distoma further back. Steenstrup, however, entertains the not unfounded supposition that the "parent-nurses" are not produced from other similar creatures, but that they proceed originally from ova derived from the full-grown Fluke, - a supposition which derives additional importance from observations made upon the development of other Entozoa belonging to the Trematode group.

(388). In Monostomum mutabile*, for example, which inhabits several of the cranial cavities lined with mucous membrane in certain water-birds, the young embryo is frequently hatched before or just as the ovum is expelled. The newly-hatched young (fig. 73,1) are elongate-oval, and furnished at their' anterior extremity with some short lobes, which the animal is able to protrude and retract; and its whole surface is covered with vibratile cilia, by the aid of which it moves readily in the water. In the anterior part of the body are two quadrangular spots, which can scarcely be regarded as anything but eyes. The posterior two-thirds of the trunk are occupied by a slightly transparent whitish body (g), which it might be supposed was one of the viscera, as Siebold thought it to be, if it were not that after a time, and some rather vigorous motions, it becomes detached, ruptures the body of its parent, and presents itself as an animal of entirely different appearance from that in which it lay concealed and was developed (fig. 73, 2,3).

(389). Now, since this enclosed animalcule is constantly present in the ciliated young Monostomum, and as there is always but one animalcule in each individual, and always in the same situation, there must necessarily be some organic connexion between the two, and one entirely different from that which it has been supposed could be explained by styling the one a necessary parasite of the other. If one animal is, organically speaking, necessarily connected with another, so that each can be developed only in or around the other, they must belong to one and the same unity, or constitute such a unity; and this is doubtless the case with the animals we are now considering.

1. First stage in the development of Monostomum mutabile.

Fig. 73. 1. First stage in the development of Monostomum mutabile, after it has quitted the egg and is swimming about at liberty: g, internal embryo of "parent-nurse." 2,3. The same, after its metamorphosis from an active form into an inactive sluggish creature, which is not itself a mother, but which nourishes within it a progeny from which, in the third generation, a parent animal proceeds. (After Siebold).

* Vide Wiegmann's Archiv fur Naturgeschichte, 1836.

(390). The Acanthocephala*, called likewise Echtnorhyncht, constitute a distinct group of Helminthozoa, distinguished by the remarkable armature of their oral apparatus, as well as by the peculiarities of their internal organization.

(391). The Echinorhynchi inhabit the intestinal canal of various animals, to the walls of which they fasten themselves by a singular contrivance. In the Echino-rhynchus gigas, which is found in the intestines of the hog, the head (a, fig. 74, 1, 2, 3) is represented by a retractile proboscis, armed externally with four circlets of sharp recurved hooks, which, when plunged into the coats of the intestine, serve as secure anchors whereby the creature retains itself in a position favourable to the absorption of food. In fig. 74,1, 2, this aculeated proboscis is represented of its natural size, relative to the body of the Entozoon, as it appears when fully protruded; but when not in use, the spinous part is retracted, and concealed by the mechanism of which an enlarged view is given at fig. 74, 3. "When extended, the position of the organ is indicated by the dotted lines; but in the drawing the whole organ is represented as drawn inwards and lodged in a depression formed by the inversion of the iritegument, so as completely to hide it within the body. This inversion is produced by the contraction of two muscular bands, d, e, which arise from the inner walls of the body, and are inserted into the root of the proboscis around the oesophagus*; two other muscles, b b, antagonistic to the former, arise near the spines themselves; and these, aided by the contractions of the walls of the body, are the agents by which the protrusion of the head is effected. Although the teeth or spines which render this organ so formidable are merely epidermic appendages, they are raised or depressed at the will of the creature; and it is therefore probable that, minute as they are, they have muscular fibres connected with them, serving for their independent motions. These spines, moreover, are not always confined to the head, but in many acanthocephalic worms are found on various parts of the body, wherever their office as instruments of attachment is by circumstances rendered needful.

Anatomy of Echinorhynchus gigas, female (after Cloquet.)

Fig. 74. Anatomy of Echinorhynchus gigas, female (after Cloquet.) 1. a, proboscis; b,c, ovarian apparatus; d d, lemnisci; e e, retractor muscles of the proboscis; ff, alimentary tubes; A, external opening of the female generative system. 2. Anatomy of male Echinorhynchus: a, proboscis; d, lemnisci; f, g, testes; h, vasa deferentia, uniting together at i; k, vesiculae seminales; 11, retractor, and n n, protruding muscles of the penis; m, their point of insertion into o, the penis; p, generative aperture. 3. Mechanism of the proboscis: a, its extremity covered with curved spines; b, c, its protractile sheath; d, e, retractor muscles; f, lemniscus; g, ovary.