500 G. H. PARKER 



called the upper face dorsal. I shall, therefore, not use the terms 

 dorsal and ventral for the parts in Renilla, but shall employ 

 superior and inferior as indicated by the natural position of the 

 animal. The face of the rachis that is upper when the animal 

 is normally at rest and that carries the zooids is superior, the 

 opposite face inferior. 



If an expanded Renilla is watched in sea-water its autozooids, 

 generally distended, will be seen to exhibit from time to time 

 spontaneous withdrawals and expansions. When one of these 

 withdraws, its eight tentacles are first folded together, after 

 which the whole zooid bends sharply to one side so as to appear 

 to be lying almost flat on the surface of the colony while it slowly 

 slips backward into the cavity occupied by it in the colony as a 

 whole. The complete withdrawal is accomplished in a few sec- 

 onds. In spontaneous expansion the eight tentacles first open 

 at the aperture in the colonial flesh into which the autozooid has 

 withdrawn and its body next slowly elongates and rises out of 

 this aperture till it becomes fully extended. The process of 

 expansion requires also only a few seconds, but is usually some- 

 what slower than that of withdrawal. 



In spontaneous withdrawal and expansion the various auto- 

 zooids seem to act with complete independence, for no unanimity 

 or sequence of response was observable among them. The in- 

 dividual autozooids are extremely inert to mechanical stimula- 

 tion. It is scarcely possible to induce them to respond even to 

 vigorous prodding. They are, however, very responsive to a 

 faradic current, withdrawing at once when this stimulus is ap- 

 plied to them. In this instance, as in spontaneous withdrawal, 

 they act with complete independence and repeated attempts to 

 influence adjacent autozooids by stimulating a given one always 

 resulted negatively. Only under particular conditions did many 

 of them respond together. When the flesh of the colony as a 

 whole contracted and the contained fluids were thus put under 

 unusual pressure, many autozooids — withdrawn but apparently 

 relaxed — expanded in unison. Muscular action also appeared at 

 times to increase the volume of the colony as a whole and, under 

 these conditions, many autozooids simultaneously withdrew. 



