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Having been acted upon by the mesenteric glands, and passed through them, the chyle flows onward until it is poured into a dilated reservoir for the chyle, known as the receptaculum chyli. This is a sac-like expansion of the lower end of the thoracic duct.
The freshwater polyp Hydra, like the foregoing illustrations, is one whose structure has already been discussed in the earlier chapters, but now we may use it for an analysis of another series of biological phenomena. Its sac-like body consists of two cell-layers; the outer one is concerned primarily with offense and defense, while the inner layer is made up of digesting or nutritive elements.
It differs from the genera already mentioned, in remaining within its egg membrane, and not assuming their strange forms. From the non-segmented, sac-like larva, it passes directly into the pupa state. The last egg-parasite noticed by Ganin, is Teleas, whose development resembles that of Platygaster. It is a parasite in the eggs of Gerris, the Water Boatman.
If these glands have the general form of tubes, they are called tubular glands; if sac-like in shape, they are called saccular glands. Both the tubular and the saccular glands may, by branching, form a great number of similar divisions which are connected with one another, and which communicate by a common opening with the place where the secretion is used.
Slowly, inexorably, that mottled tentacle curled downward with its prey, and a portion of the under side of the rock became alive! Two ink-black eyes appeared, bulging, oval, implacable; and between them opened a great, hooked beak, like a giant parrot's. There was no separate head behind this gaping beak, but eyes and beak merely marked the blunt end of a mottled, oblong, sac-like body.
Although the microscope made impossible this very simple explanation, it gave in return a truer, if more complex, account of the transformation from egg to offspring. By this means it has been definitely proved that the ovum multiplies rapidly after it has been fertilized, and becomes, as was explained in the preceding chapter, a sac-like structure within which hangs a tiny clump of tissue.
The most common of the group, the C. acaule, most widely known as the moccasin-flower, whose large, nodding, pale crimson blooms we so irresistibly associate with the cool hemlock woods, will afford a good illustration. The lip in all the cypripediums is more or less sac-like and inflated.
And behind them those two great, inky, impassive eyes, and then the fat, mottled, sac-like body, emerged furtively from under the rock. The bottom, just at this point, was covered with a close brown weed, and almost at once the body of the octopus and his tentacles began to change to the same hue. When the change was complete, the gliding monster was almost invisible.
The non-striated muscle is found in the walls of the food canal, blood vessels, air passages, and other tubes of the body; while the muscular tissue of the heart is confined entirely to that organ. Because of their great length they are called fibers, or fiber cells. A thin sac-like covering, called the sarcolemma, surrounds the entire cell and just beneath this are a number of nuclei.
Then it commences to undergo remarkable changes and finally becomes a mere sac-like organ with a number of long slender root-like processes penetrating and taking nourishment from the body of the unfortunate crab-host. The worms furnish many well-known examples of parasites, whole groups of them being especially adapted to parasitic life.
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