ON THE LAB0RAT0E7 OF THE MAEINE BIOLOGICAL ASSOCIATION. 361 



The ectoderm is in the uninjured specimens composed of several 

 layers of cells. Under unhealthy conditions the superficial layei's are 

 apt to slough. 



The stomodfeum of each fully-developed polype opens into a long 

 coolenteron that passes down to the base of the colony. In a longitudinal 

 section through a branch of the colony the 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 (?) netwoi'k can only be made out in fresh osmic acid 

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

 isolated mono-, bi-, and tri-polar cells. It is difficult to trace in the 

 peripheral parts of the colony, the presence of a very large number of 

 spicules making it very difficult to cut thin sections of the fresh 

 tissues. 



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

 Tiibipora, Glaviblaria, Sarcophytum, the Astrteidfe, and a few others, 

 remain expanded until the water actually leaves tbe reefs. Others, on 

 the other hand, such as Heliojjora, Millepora, 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 occun-ed to me that this might be due, to a certain extent, 

 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 supposi- 

 tion I made last autumn a series of experiments upon Alcyonium digita- 

 tum in the tanks at the Plymouth Laboratory. 



I placed a number of sjsecimens of Alcyonmm digitatum, collected 

 partly in the shallow water of the Catwater and partly in deeper water 

 ofi' the Eddystone Lighthouse, in one of tbe 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 putrefy. 



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

 arranged an artificial tide by means of syphons. It was so arranged that 

 the water should run off — but not so completely as to leave the Alcyo- 

 niums uncovered — in twelve hours and fill up again in twelve hours. 

 The Alcyoniums contracted with tolerable 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 a healthy condition contracted regularly only once 

 in twenty-four hours. 



These experiments appear to indicate, first, that there is a rhythmic 

 •contraction of the polypes of Alcyonium digitatum in the normal condi- 

 tions twice in every twenty-four hours ; second, that a new rhythm may 



