29 



place by the pressure caused by retraction of the mouth-mem- 

 brane. 



Several samples were studied a little more closely. Some 

 contained practically nothing but sand and some detritus, others 

 however were full of interesting substances, most of which 

 could easily be identified. Some were full of calcareous material, 

 as a foaming with hydrochloric acid proved; others did not 

 contain anything of the kind. 



As examples of materials which I could easily identify, I may 

 mention the following constituents. 



Several species of diatoms were seen. Some of these moved 

 around very actively. Though I tried to avoid all contamination 

 with the sea water of the aquarium, some planctonts may have 

 come into the material; for this reason I am not inclined to 

 consider this observation as very important. 



Otherwise it would be a very convincing demonstration of 

 the weakness of the action of the digestive juice. Some unat- 

 tacked unicellular algae were also found. Sand-grains are present 

 in nearly every sample. Brittles, maybe of Bryozoa, are abundant. 

 A hydroid was found (Sertularia?). In some of the compart- 

 ments a seemingly intact, but contracted individual was seen. 

 Spiculae were seen, probably derived from sponges. Red algae, 

 also seemingly intact, frequently occurred. The granulation of 

 the cell-contents and the chromatophores were clearly visible. 

 It was a pluricellular branch : it did not seem to have been 

 attacked severely. 



Peculiar honey-comb-like structures, like minute plant-tissue 

 (Cyanophyceae ?), could not be identified. A leg of a copepod 

 seemed to be attacked and the flesh dissolved away. 



From this evidence we are once more impressed of the relative 

 weakness of the enzymes present in the free form in this group. 

 Of the large amounts of material which pass through the gut 

 only a very small part can be attacked succesfully, some sub- 

 stances pass without serious changes. If really alive organisms 

 are present, this is the most convincing proof; this does not 

 seem to be very probable to me, though. ') 



As far as the feeding habits of the species are concerned, 

 we see that it is practically omnivorous. Most probably it eats 

 nearly everything, chews algae, also calcareous species, sponges, 

 Bryozoa etc., by means of its strong masticating apparatus, and 

 in this way also gets hold of its microscopical inhabitants and 

 some other materials. The only thing which I actually observed 

 an Arbacia eating, was Clione sulfurea, a calcisponge. 



l ] The same incompleteness of the digestion has been found by Plateau 

 (Recherches sur les phenomenes de la digestion chez les insectes. Bruxelles. 

 1 874) in catterpillars and by Knauthe (Untersuchungen uber den Stoffwechsel 

 der Fische. Zeitschr. f. Fischerei. 5. 1897. 189) in fishes. 



