28 



Apart from numerous sand-grains many pieces of decaying 

 plant-tissue were found, recognisable by the cell-walls and by 

 their brownish appearance. On the other hand however I 

 could frequently see diflFerent species of unicellular green algae, 

 which were perfectly intact and still in the possession of green 

 chloroplasts, a nucleus and all other structural details. . Many 

 empty diatom shells were present, also a flagellate species, 

 empty, with cirri sticking out. Many diatoms however were 

 perfectly intact just as the green algae. Encysted Protozoa (?), 

 perfectly round bodies, also frequently occurred. A complete 

 copepod was seen, not moving, but very complete up to the 

 smallest brittles, the eye full of pigment. Nothing gave the 

 impression of being dissolved or dissolving, the heart was not 

 beating. Pieces of other Crustaceans were also found, some- 

 times nothing but the chitinous sceleton. 



Large quantities of detritus were present in another animal 

 of which I studied the contents of the rectum. Here also 

 complete green algae, uni- as well as pluricellular forms were 

 found. Diatoms, either empty shells or in full possession of 

 all the cell constituents, green algae, thread-like and with 

 contracted cytoplasm — as if in a hypertonic solution — , 

 were also very abundant. Pluricellular red algae with contracted 

 cytoplasm, but uninjured and in full possession of the natural 

 colors were also found occasionally. 



To this list P e a r s e 98) still adds nematodes and an ostracode. 

 In every animal I dissected many alive Protozoans were found 

 in the rectum, moving around with great speed in the visceral 

 fluid. According to Pearse they belong to the genera Gym- 

 nodinium and Lichnophora. 



Ulva-leaves digested with the digestive juices of Arbacia 

 and Thyone did not seem to be very much attacked. Some- 

 times the cells at the outer margin had a slight brown color, 

 the whole leaf, even after digestion at 37° for one or two 

 days, remained green however and kept all its structures. 



10. COMPOSITION OF THE FAECES OF ARBACIA. 



A study of the faeces of Arbacia was made with a double 

 purpose. In the first place it may give us an idea as to the 

 intensity of action of the enzymes in the intestine, secondly it 

 may give us some information about the feeding habits of this 

 species. 



The faeces of this species are most easily secured. Shortly 

 after the animals have been brought into the laboratory, their 

 back is as a rule covered all over with small, oval bodies of 

 a white color which prove to be their faeces. Even after 5 — 6 

 days the gut is still always not empty and faeces occasionally 

 produced. According to von Uexkiill 128) defecation takes 



