No abstract

Of the relation of amœboid movement to other forms of contractility, very little is known at present. Hypotheses have been advanced to explain the movement, but they differ widely among themselves, and are founded almost entirely on direct observations of the normal activities of amœba. More recently Loeb (24) (25) and others have tried to determine the rôle of various environmental factors, such as the presence of certain salts, in amœboid activity. It is on these lines that the present work is being conducted.

The Diplopoda have exploited the ' bottom gear ' type of Onychophoran gait, the emphasis being on a powerful rather than a speedy gait. With the long backstroke is associated a short segment, and a frequent development of a large number of body segments. The formation of diplosegments by fusion provides a mechanism counteracting waste of effort by dorso-ventral undulations at the higher speeds. The Symphyla resemble the Diplopoda in many respects, but differ from them in others. The Geophilomorph and Scolopendromorph Chilopoda have elaborated the ' middle gear ' gait of Peripatus, ultimately gaining considerable speed by a quickening of the backstroke. This gait is associated with a more elongated segment, and where the legs are fairly long and the backstroke very short, an alternation in the size of the tergites reduces the tendency towards lateral undulations which are wasteful of effort. Some Diplopoda have secondarily increased their speeds and developed a facies approaching that of the Chilopoda in some respects, while a few Chilopoda appear to have become content with slow locomotion and have correspondingly assumed some Diplopod-like features in association with their gait. The Lithobiomorph and Scutigeromorph Chilopoda have progressed further than the Scolopendromorpha in the evolution of speed, but in an entirely different manner, combining the ' top gear ' gait of Peripatus with much longer legs. The longer legs introduce many problems which are met by a shortening of the segment and a reduction in the number of segments ; and an increased tendency towards lateral undulations is countered by an increase in the difference between the lengths of the tergites and by their fusion together as an Sczltigera. The Pauropoda show much in common with the Scolopendromorpha and Insecta, and like them must have originally adopted a gait resembling the 'middle gear' of Peripatus. Time did not permit of a further consideration of other groups of Arthropoda, but it may be said that the Arachnida, Insecta, and many higher Crustacea gain speed mainly by an increase in the length of the leg, as in the Lithobiomorpha and Scutigeromorpha, but they have gone further than these chilopods in solving the common problems resulting from longer legs. Some of the difficulties dependent upon the possession of many long legs can only be eliminated if a smaller number of long legs are situated anteriorly on consecutive segments which are either fused together or lack mobility in certain planes. These features, together with the tendency to reduce or eliminate the segments at the posterior end of the body, are common to the Arachnida, Insecta, and Malacostraca. With a reduction in the number of walking legs, certain changes in the gaits become inevitable, and some gaits appear which are not performed by any of the many-legged arthropods. The correlation between the morphology and the gaits employed by the various groups of Arthropoda indicates that these animals must have evolved independently for a very long period of time, perhaps si...

4~ upper littoral nemertine from Portobello. New Zealand, shows relationships with certain Geonemertes species, but is differentiated from them. A new genus Acleonemerles is made for the reception of the species which is given the name of Acteonemerles balhamae.

1. In constant environmental conditions Metridium senile exhibits continual slow inherent activity. The pattern of this activity varies in character from time to time. These different patterns of activity have been termed ‘phases’. A particular phase may endure for hours on end and then rather quickly give place to another phase. 2. A change of phase may be initiated by certain stimuli. Ingestion of food initiates a sequence of phasic changes involving expansion of the disk and elongation, followed by swaying, parietal contraction, distension, defaecation and ‘shrivelling’. Each of these phases has its own pattern of activity. Not all may be exhibited by any one animal, or by the same animal at different times. A similar sequence of phases to those induced by solid food may be initiated by mere temporary exposure to filtered food solution. 3. In contrast with the direct responses, such as the retraction reflex, the stimulus does not directly maintain a phasic response; it merely initiates a new phase, and the activity pattern of this new phase is maintained long after (even hours after) the initiating stimulus. This relation of phase change to stimulus may be evident after electrical stimulation as well as after exposure to food or other stimuli. 4. Phase changes also differ from simple reflexes in that the threshold for their initiation varies enormously in different animals and in the same animal at different times. Also the threshold may sometimes be so low that the phase occurs spontaneously in the absence of evident external stimuli. Changes of threshold cannot be adequately accounted for by sensory adaptation. The stimulus apparently acts by releasing a complex activity pattern (the phase) which is so far independent of the stimulus that it may appear spontaneously. 5. Locomotion is another phasic activity. It is a complex co-ordinated activity pattern. It may be initiated by various stimuli. The threshold varies at different times and in different animals. It may take place spontaneously in the absence of evident external stimuli. The threshold of this phase is lowered after feeding and raised by illumination. 6. Alternating phases of expansion and contraction frequently occur in Metridium. Their relation to diurnal and other rhythms is discussed. Daily illumination can often initiate and control regular daily phases of contraction. This is true both of daylight and of periodic exposure to artificial light. As with other phase changes, the threshold varies greatly. In some cases each periodic illumination may only induce a brief temporary contraction and fail to control phase change. 7. In complete darkness and constant environmental conditions, alternating phases of expansion and contraction may still take place. These may be irregular, but sometimes they may assume a very rough rhythm. When present, this rhythm does not keep in step with previous daily stimuli, nor with current external changes of day or night, nor with other environmental rhythms. The unstimulated animal thus possesses an inherent tendency to alternating phase change which may approach a rhythm. It appears that periodic stimuli, such as daily illumination, act by ‘setting the pace’ of this inherent alternating phase change. 8. Different phases which involve the same groups of effectors may reinforce or may conflict with one another. 9. Our experiments show that continual and varying patterns of inherent activity play an important part in the behaviour of Metridium. The behaviour is not simply a succession of direct reflexes to stimuli acting on a passive animal. Such direct responses are, however, more easily observed because phasic activity is extremely slow. Phasic activities play an essential part in behaviour patterns such as food capture. They are often behaviouristically relevant to a future possible event rather than to a past stimulus; as when sweeping and swaying movements increase the chance of finding food. The relation of phases to the ‘physiological states’ of Jennings is noted.