. Discovery reports. Discovery (Ship); Scientific expeditions; Ocean; Antarctica; Falkland Islands. Text-fig. 1. Diagram of Physalia, seen from above, to show the sailing posture. M = direction of movement, W = wind direction, a = apical pore end of float, b = bulge, c = oral end, 5 = sail (crest). The arrows around the bulge (b) represent the forces of drag caused by the tentacles. The way in which the wind acts has not been determined. Nerve-cells equipped with sense-hairs (to be described below) occur in the ectoderm of the float, and may be concerned in the reception of wind stimuli. Alter
. Discovery reports. Discovery (Ship); Scientific expeditions; Ocean; Antarctica; Falkland Islands. Text-fig. 1. Diagram of Physalia, seen from above, to show the sailing posture. M = direction of movement, W = wind direction, a = apical pore end of float, b = bulge, c = oral end, 5 = sail (crest). The arrows around the bulge (b) represent the forces of drag caused by the tentacles. The way in which the wind acts has not been determined. Nerve-cells equipped with sense-hairs (to be described below) occur in the ectoderm of the float, and may be concerned in the reception of wind stimuli. Alternatively the buffeting and agitation caused by the wind or spray, or a combination of such factors may evoke direct responses in the float musculature, leading to crest erection. It should be explained that erection of the crest occurs automatically whenever the float musculature achieves a certain degree of tension. Any stimulus evoking an overall tightening of the float muscula- ture will therefore normally lead to crest erection. The mechanism is surprisingly simple. The float consists of two containers: an outer muscular case, the codon (strictly:' pneumatocodon') and an inner air bladder, the saccus ('pneumatosaccus'). The saccus lies free in the coelenteron, being surrounded by the codon and attached to it only in the region of the apical pore. The saccus is an inert, pliable structure whose shape is determined by that of the enclosing codon. Electric shocks applied to its wall were found to evoke no visible response, either local or general. Along its upper side it bears a row of thin-walled outpushings which become inflated whenever the air in the saccus as a whole is sufficiently compressed. At other times, the outpushings are collapsed and inconspicuous. This can be demon- strated experimentally by removing a saccus intact from a specimen, ligaturing its apical connection with the codon as a precaution against loss of air, and placing it under pressure by means of wei
Size: 1585px × 1577px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, booksubjectocean, booksubjectscientificexpediti
