1892.] anatomy and habits of Alcyonium digitatum. 307 



coelentera spread from the base in a fan-like manner towards the 

 periphery. 



Between the coelentera there is a dense clear mesogloea 

 containing spicules, endodermic cell cords, a very delicate network 

 of nerve (?) fibrils and cells. 



The young polypes originate in these endodermic cords, the 

 coelentera being developed later, and do not communicate directly 

 with the coelentera of the older polypes until they are nearly full 

 grown. 



The endodermic cords are usually described as canals but there 

 is no evidence of the presence of a lumen. Numerous injection 

 experiments failed to prove the existence of any cavity in these 

 structures. 



The nerve (?) netwoik can only be made out in fresh osmic 

 acid preparations. It consists of a fine network of delicate fibrils 

 connecting isolated mono- bi- and tripolar cells. It is difficult to 

 trace in the peripheral parts of the colony, since the presence of 

 a very large number of spicules makes it a matter of some diffi- 

 culty to cut very thin sections of the fresh tissues. 



When the tide is low in the tropics some forms of Zoophytes, 

 such as Tubipora, Clavularia, Sarcophytum, the Astraeidse and 

 a few others, remain expanded until the water actually leaves the 

 edge of the reef. Others on the other hand, such as Heliopora, 

 Millepcra and most of the Madrepores, completely retract while 

 there is still a foot or more of water covering them. Some 

 Zoophytes in fact appear to anticipate low tide before others, and 

 it occurred to me that this might be to a certain extent due to 

 the development of a rhythm similar to the rhythmic movements 

 of certain plants. 



In order to determine if possible the truth or falsity of this 

 supposition I made last autumn a series of experiments upon 

 Alcyonium digitatum in the tanks at the Plymouth laboratory. 



I placed a number of specimens of Alcyonium digitatum, 

 collected partly in the shallow water of the Catwater and partly 

 in deeper water off the Eddystone lighthouse, in one of the tanks, 

 and I noticed that nearly all of them contracted completely twice 

 in every twenty-four hours for the first three or four days. 



Those that did not contract in this manner soon showed 

 bubbles of gas in their tissues and commenced to putrify. 



I also placed a number of specimens in another tank in which 

 I arranged an artificial tide by means of siphons. It was so 

 arranged that the water should run off — but not so completely 

 as to leave the Alcyoniums uncovered — in twelve hours and fill 

 up again in twelve hours. The Alcyoniums contracted with toler- 

 able regularity twice in twenty-four hours for the first two days 

 and then contracted quite irregularly, some only once, some twice. 

 At the end of a fortnight two out of the three that remained in 



