CONTROL 125 



lining the tube. The albuminous axis is usually regarded 

 as food material, but it might well be in reality a path for 

 nervous impulses. Closely applied to the tubes one finds 

 certain peculiar cells called " companion cells " whose 

 functions are unknown. It is tempting to wonder if they 

 might not be the plant's nerve-cells. At intervals the sieve- 

 tube is partitioned by perforated plates across which at 

 times the protoplasmic lining spreads, thus breaking the 

 continuity of the albuminous axis ; and although we do 

 not for a moment suggest that the medullated nerve-fibre 

 of the vertebrate owes anything to plant-life, yet we cannot 

 but notice a certain similarity of design in this structure. 

 For it has a central conducting axis-cylinder surrounded 

 by a " medullary sheath," and is partitioned at intervals 

 by a plate, or " node of Ranvier," through which the axis- 

 cylinder alone seems to pass. It is just possible that this 

 may be a case of parallel design. 



Turning now to the Discontinuous megazooid — aD Indi- 

 vidual as is the Sea-anemone, or part of one as is the medusi- 

 form gonophore — we meet again a diffuse communicating 

 nervous layer bringing all parts of the organism into touch 

 with each other. This is what we would expect, for the 

 megazooid is equivalent to a number of zooids fused in 

 lateral continuity round a central stem, and it is natural 

 that the ancestral zooidal nervous layers should be linked 

 up in continuity. At the same time the nervous layer of 

 the megazooid is more complex than that of the zooid, as 

 will be seen in the figure below (Fig. 45). 



In medusiform organisms, however, there is in addition 

 to the diffuse nervous system mentioned a special one whose 

 presence is undoubtedly connected with the function of 

 bell-contraction. For round the rim of the bell there are 

 situated at intervals special nerve-ganglia linked together 

 by a circular nerve-cord. Each ganglion is a little control- 

 station governing a special section of contractile bell ; and 

 we have here the first appearance of a definite nervous 

 system on a concentrated repeating plan, and which, more- 

 over, is probably the basis of the nervous system of the 

 highest forms of living Continuity. The circular nerve- 

 cord with its ganglia is clearly for the regulation of the 

 contractions and relaxations of the medusoid swimming- 



