. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 168 EMIL ZUCKERKANDL paratively low pressures the progression was never fast. High speed progression is not possible before the animal is wholly under sand. Indeed, the "hump" is effective under these conditions only as explained below. Since fast progression requires high pressure and is possible only when the animal is nearly completely under sand, pressure gradually increases as it becomes effective. ANIKM UNDEH SAND "PRESSURE of 90. ZERO LINE TIMING PROBOSCIS Acnvrry FIGURE 5. Sipunculus nudus, coelomic pr
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 168 EMIL ZUCKERKANDL paratively low pressures the progression was never fast. High speed progression is not possible before the animal is wholly under sand. Indeed, the "hump" is effective under these conditions only as explained below. Since fast progression requires high pressure and is possible only when the animal is nearly completely under sand, pressure gradually increases as it becomes effective. ANIKM UNDEH SAND "PRESSURE of 90. ZERO LINE TIMING PROBOSCIS Acnvrry FIGURE 5. Sipunculus nudus, coelomic pressure and activity recorded every third second with aid of metronome. Conventions for proboscis activity as in figures 1 and 3. Time line, 10 second intervals. Figure 6 shows how pressure variations and body movements are coordinated during burrowing under sand. The results are similar to those for burrowing movements in water except that the maxima often reach very much higher values. The period of low maxima corresponds to one of lower burrowing activity; the animal progresses less speedily and remains longer in the "feeding position" (pro- boscis and tentacles stretched out). Under sand, proboscis protraction has been seen to continue under a pressure as low as cm. As in animals kept in free water, tentacle eversion was brought about by a retrac- tion of the distal section of the proboscis. In this way the tentacles are set free within a cavity prepared beforehand by the burrowing proboscis and are protected from rough contacts with the sand. Figure 5 affords evidence that the normal correspondence between burrowing cycles and pressure cycles may be systematically upset during burrowing in sand, just as it was occasionally during movements in free water. One burrowing cycle consistently corresponds to two pressure cycles. (The movements could not be fol- lowed during the later part of the record.) Proboscis retraction does not occur. Please note that these ima
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