Читать книгу Jelly-Fish, Star-Fish, and Sea-Urchins: Being a Research on Primitive Nervous Systems - George John Romanes - Страница 9

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The next question which naturally presents itself is as to whether the locomotor centres are equally distributed all round the margin of a swimming organ, or situated only, or chiefly, in the so-called marginal bodies. To take the case of the naked-eyed Medusæ first, it is evident that in most of the genera, in consequence of the intertentacular spaces being so small, it is impossible to cut out the marginal bodies (which are situated at the bases of the tentacles) without at the same time cutting out the intervening portions of the margin. The genus Sarsia, however, is admirably adapted (as a glance at Fig. 1 will show) for trying the effects of removing the marginal bodies without injuring the rest of the margin, and vice versâ. The results of such experiments upon members of this genus are as follow.

Whatever be the condition of the individual operated upon as to freshness, vigour, etc., it endures excision of three of its marginal bodies without suffering any apparent detriment; but in most cases, as soon as the last marginal body is cut out, the animal falls to the bottom of the water quite motionless. If the subject of the experiment happens to be a weakly specimen, it will, perhaps, never move again: it has been killed by something very much resembling nervous shock. On the other hand, if the specimen operated upon be one which is in a fresh and vigorous state, its period of quiescence will probably be but short; the nervous shock, if we may so term it, although evidently considerable at the time, soon passes away, and the animal resumes its motions as before. In the great majority of cases, however, the activity of these motions is conspicuously diminished.

The effect of excising all the marginal tissue from between the marginal bodies and leaving the latter untouched, is not so definite as is the effect of the converse experiment just described. Moreover, allowance must here be made for the fact that in this experiment the principal portion of the "veil"[7] is of necessity removed, so that it becomes impossible to decide how much of the enfeebling effect of the section is due to the removal of locomotor centres from the swimming-bell, and how much to a change in the merely mechanical conditions of the organ. From the fact, however, that excision of the entire margin of Sarsia produces total paralysis, while excision of the marginal bodies alone produces merely partial paralysis, there can be no doubt that both causes are combined. Indeed, it has been a matter of the greatest surprise to me how very minute a portion of the intertentacular marginal tissue is sufficient, in case of this genus, to animate the entire swimming-bell. Choosing vigorous specimens of Sarsia, I have tried, by cutting out all the margin besides, to ascertain how minute a portion of intertentacular tissue is sufficient to perform this function, and I find that this portion may be so small as to be quite invisible without the aid of a lens.

From numerous observations, then, upon Sarsia, I conclude that in this genus (and so, from analogy, probably in all the other genera of the true Medusæ) locomotor centres are situated in every part of the extreme margin of a nectocalyx, but that there is a greater supply of such centres in the marginal bodies than elsewhere.