(414). The median vascular trunk, throughout the greater part of its course, is situate immediately beneath the subcutaneous muscular layers; but when it arrives at the sheath of the proboscis it becomes enclosed in a special canal, and thus penetrates into the cephalic region, where it bifurcates to form the two lateral trunks, k k, which, after forming almost a complete circle around the cerebral ganglia, anastomose with the lateral vessels, i i.

Styliferous apparatus of Folia mandilla: a, portion of proboscis.

Fig. 77. Styliferous apparatus of Folia mandilla: a, portion of proboscis; b, portion of the intestine; c, first oesophageal dilatation; d, second oesophageal dilatation; e e, oesophageal canal, presenting sundry dilatations and contractions; f, cavity in which is lodged the spiculum, g; h h, poison?-glands in connexion with the spiculum, g; i i, cavities enclosing incompletely formed spicula.

(415). The vessels of the body thus becoming conjoined at the points h h, form a loop, g g, which surrounds the cephalic region, and is of considerably greater calibre than the trunks from which it is derived.

(416). The fluid contained in the vascular system above described is generally colourless, but sometimes of a reddish or yellowish tinge; it is, however, completely devoid of blood-corpuscles, resembling, in this respect, the blood of the Annelidans, which will be desciibed on a future occasion.

(417). In these worms, the central or abdominal space is separated from the two lateral cavities by vertical septa, to which the reproductive organs are attached. The latter consist of a series of caeca (fig. 76, n n), which are so exactly alike in the two sexes, that it is impossible to distinguish the male from the female, except by their contents. These caeca are apparently formed of several layers of cells, and are covered externally with vibratile cilia. Except during the period of reproduction, they contain, in both sexes, nothing but an opaline fluid; but when called into action, the ovaria in the female are found to be filled with a liquid wherein corpuscles of various shapes are seen to be suspended. Some of these have the appearance of perfectly diaphanous spheres, which are sometimes isolated, at others surrounded with granulations; and likewise oil-drops of a beautiful golden colour may be detected: but the ovaria never seem to contain perfectly developed ova; these are only met with in the lateral cavities of the body, where they may frequently be seen, in different stages of development, floating about between the caeca. The vitellus and Purkinjean vesicle are always more or less apparent.

When these eggs have acquired their full growth, they stuff, so to speak, the whole body of the worm, pressing upon the alimentary canal to such a degree that it seems in danger of becoming atrophied, and almost completely effacing the median abdominal cavity; nay, so closely are they squeezed against each other, that they lose their spherical form and become polygonal. The number of these eggs is prodigious; and M. de Quatrefages estimates seven or eight thousand to be a moderate complement for a middle-sized worm of this description.

(418). The male Nemerteans present phenomena very similar to those just described as occurring in the female. At the period of reproduction, the testicular caeca become filled with granulations of various sizes, sometimes isolated, sometimes grouped together in round masses; but these do not contain spermatozoids. The latter seem only to occur in the lateral cavities of the body; and even there they are found in various stages of development, from granular masses, such as are met with in the ovaria, to aggregations of spermatozoids provided with tails and subsequently isolated. At length the body of the male Nemertean appears to be as completely stuffed with spermatozoids as that of the female is with eggs, and they are ultimately expelled, in prodigious numbers, into the surrounding water.

(419). The CŒlelmintha, or cavitary intestinal worms of Cuvier, evidently present a much higher type of structure than any of the preceding. The Ascaris lumbricoides, as its name imports, so strongly resembles some of the Annelida in its external configuration, that the zoologist who should confine his attention to outward form alone, might be tempted to imagine the affinities connecting them much stronger than a comparison of their anatomical relations would sanction. This Entozoon is found in the intestines of many animals, and is endowed with some considerable capability of locomotion, adapted to the circumstances under which it lives; for in this case the worm, instead of being closely imprisoned in a circumscribed space, may traverse the entire length of the intestines in search of a convenient locality and suitable food.

(420). In accordance with such an enlarged sphere of existence, we observe muscular fibre distinctly recognizable in the tissues that compose the walls of the body - not as yet, indeed, exhibiting the complete characteristics of muscle as it is found in higher animals, but arranged in bundles of contractile filaments, running in determined directions, and thus capable of acting with greater energy and effect in producing a variety of movements.

(421). In this rudimentary state, the muscular fibre does not possess the density and firmness which it acquires when completely developed; it has, when seen under the microscope, a soft, gelatinous appearance, apparently resulting from a deficiency of fibrin in its composition; the transverse striae, usually regarded as characteristic of the muscular tissue of the more perfect animals, are not yet distinguishable; and the individual threads are short, passing over a very small space before they terminate. On examining the arrangement of these fasciculi, they are seen to be disposed in two layers, in each of which they assume a different course; thus, in the outer layer they are principally arranged in a longitudinal direction, while the inner stratum of fibres is placed transversely, affecting a spiral course, so as to encircle the viscera. From this simple structure various movements result: by the action of the longitudinal fasciculi the whole body is shortened; by the contractions of the spiral layer an opposite effect is produced; or by the exertion of circumscribed portions of the muscular integument, lateral flexions of the body are effected in any given direction.