. The Cambridge natural history. Zoology. PHYSIOLOGY OF NERVOUS SYSTEM 523 the stored tone spreads upwards to the spines, causing the weak form of spine reaction, and the spines converge. It will be seen therefore that the so-called central nervous system of Echinus does not act in any sense as a brain, as indeed might have been guessed from the absence of any differentiation in it. As Uexklill points out, when an animal is covered all over with similar organs, such as spines and pedicellariae, capable of acting automatically, a brain is not needed. The object of a brain is to direct organs wh
. The Cambridge natural history. Zoology. PHYSIOLOGY OF NERVOUS SYSTEM 523 the stored tone spreads upwards to the spines, causing the weak form of spine reaction, and the spines converge. It will be seen therefore that the so-called central nervous system of Echinus does not act in any sense as a brain, as indeed might have been guessed from the absence of any differentiation in it. As Uexklill points out, when an animal is covered all over with similar organs, such as spines and pedicellariae, capable of acting automatically, a brain is not needed. The object of a brain is to direct organs which are in a certain place Fig. 233. To show character and distribution of the sphaeridia in Strongylocentrotus droebachiensis. A, a portion of a radius, with sphaeridia, and the adjoining edge of the peristome, p, Pair of pores for a tube-foot; per, peristome ; t, primary tubercle. B, an isolated sphaeridiiim. (After Loven.) to a danger which may come from any quarter, but in the Sea- urchin any spine is as good as any other spine, and such orienta- tion is not needed. " In a dog the animal moves its legs, in a Sea-urchin the legs move the ; What the Sea-urchin does need is a means to prevent its pedicellariae attacking its own organs with which they may come into contact. Thus it possesses an " autodermin," a chemical contained in the ectoderm which paralyses the muscles of the pedicellariae, as may be seen by offering to them a spine of the same animal. If, however, the spine be treated with boiling water, and then offered, it is viciously seized, showing that this substance can be dissolved out. Just as in the case of the Starfish, when the nerve-ring is. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harmer, S. F. (Sidney Frederic), Sir, 1862- ed; Shipley, A. E. (Arthur E
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