. An introduction to zoology, with directions for practical work (invertebrates). COELENTERA TA 27. a long, whip-like, spirally-coiled filament, which lies supported in the liquid which fills the bag (Fig. 12, A). Surrounding the whole structure, but lying chiefly to one side of it, is the protoplasmic cell from which it has been formed. When the dart cell has reached the surface of the skin, there projects from it a tiny proto- plasmic thread known as the " trigger hair " (Fig. 12, t), for when this is touched the dart cell immediately "explodes," flinging out the thread a
. An introduction to zoology, with directions for practical work (invertebrates). COELENTERA TA 27. a long, whip-like, spirally-coiled filament, which lies supported in the liquid which fills the bag (Fig. 12, A). Surrounding the whole structure, but lying chiefly to one side of it, is the protoplasmic cell from which it has been formed. When the dart cell has reached the surface of the skin, there projects from it a tiny proto- plasmic thread known as the " trigger hair " (Fig. 12, t), for when this is touched the dart cell immediately "explodes," flinging out the thread as in Fig. 11, B. The thread enters the skin of the victim, carrying with it, apparently, some poison, for it has a paralysing efiect on such small prey as water-fleas, etc. Some of these dart cells have little recurved barbs at the base of the thread, but these are absent in the smallest darts. Such specialised structures as these dart cells show a great advance on anything found amongst the simple organisms of the Protozoa. Close to some of these dart cells have been found small, very irregular cells, with large nuclei, resembling the nerve cells of higher forms, and it is probable that in Hydra they function as a very rudimentary nervous system. Digestive The digestive cells, which form the inner layer Cells. of the body-wall, are less specialised, though they are considerably larger than the skin cells; in some ways they resemble the primitive amoebifdrm cell. They lie closely packed, side by side, except for their inner ends, which pro- ject freely into the body-cavity and are very irregular in shape, some ending in blunt processes, and others in whip- like threads, which may at any moment be withdrawn, and a blunt process projected instead. These cells have, therefore, to some extent, a power of individual movement comparable to the movement of some Protozoa. Digestion may take place in two ways. Some of the digestive cells seem to be specially glandular, and they secrete
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Keywords: ., bookcentury1900, bookdecade1910, bookpublisherlondo, bookyear1913
