408 H. BOSCHMA. 



that of a form without" these organisms. Now Astrangia dance 

 generally is completely devoid of zooxanthellae, but during the 

 time I worked in Woods Hole I obtained some colonies which were 

 strongly infected with these algae. The fact that my material 

 consisted partly of polyps without zooxanthellae and partly of 

 those infected with the symbiotic algae enabled me to make a 

 comparison of the feeding and digestion in these different kinds of 

 polyps. As I disposed during the first half of my time in the 

 Marine Biological Laboratory only of polyps without zooxan- 

 thellse, the larger part, of my experiments were made with such 

 uninfected polyps. Afterwards, when comparing the phenomena 

 of digestion in polyps without zooxanthellae with those found in 

 uninfected polyps, I could at least partly investigate the role of 

 the zooxanthellse in the feeding of the polyps which are infected 

 by these algae. 



I kept the polyps of Astrangia easily alive in glass-vessels with 

 running sea-water, and at the end of a month they were in the 

 same healthy state as at the beginning of the experiments. Some 

 colonies even had enlarged in size by budding. The fact that 

 Astrangia may be kept alive for some time in artificial sur- 

 roundings was already recognized by Agassiz (1850), who kept his 

 specimens alive by changing the sea-water twice a day. This 

 author also gives a description of the structure of the polyps and 

 states that they can be fed with fragments of molluscs. Besides 

 this he gives some notes on the digestion in Astrangia. According 

 to Agassiz the food at first remains for some time in the upper 

 part of the gastric cavity before it descends to the lower part of 

 this cavity where it is mixed with water and distributed over the 

 internal organs of the polyp. As may be seen in the following 

 pages these notes on the digestion are not completely in ac- 

 cordance with my observations. 



The description of the general form of the polyps by Agassiz is 

 very accurate. He states that there are three cycles of tentacles, 

 those of the first cycle are of a larger size than those of the second, 

 whilst the tentacles of the third cycle are somewhat smaller than 

 the others. At each extremity of the laterally compressed mouth 

 a tentacle of the first cycle is found. Agassiz discerned two 

 varieties in the species: one with white polyps and another in 

 which the polyps were pink or rosy. 



