39 
poda and Schuzopoda and then become of a better taste and also change in their 
outer appearance. This animal is thus as adult sometimes detritus-feeder, some- 
times predatory. In this Ehrenbaum is undoubtedly right; his careful investiga- 
tions might have made both Rauschenplat and Eichelbaum careful in drawing 
conclusions from stomach investigations on a material collected incidentally for 
other purposes. Their subdivision of animals into predatory, detritus-feeders etc., 
must therefore also be treated with care; — the boundaries are very often deleted 
in nature. 
In the »Report for 1899 on the Lancashire Sea-Fisheries Laboratory, 1900« 
J. Johnstone describes, p. 36, what Cardium edule contains in its stomach, namely, 
in addition to sand and fine 00ze, which is the principal mass, spores, stages of 
lower algae, Foraminifera, diatoms and small microscopic Crustacea; which of 
these is their principal food, he does not mention. In an earlier Report he has 
stated, that nothing is found as a rule in the digestive tract of Cardium, it is quite 
empty. I mention this, because I have found a similar condition in Buccinum (see 
below). Å very extensive literature might be cited on the stomach contents of 
marine animals; but I do not think we should get any further in the matter in 
this way, and I shall therefore go over to a discussion of my own investigations 
on this subject. 
To obtain accurate knowledge regarding the food of the animals, I have 
preferred to investigate a restricted animal community, and have taken as 
special basis for these investigations one of the most restricted divisions of the 
Limfjord, Thisted Bredning. As the tables in the next section will show, an 
insight has been gained with regard to the mass per m.? of each single species, 
so that we are able, at least for the part lying outside the plant belt, the soft clay 
bottom, to determine both by number of individuals and by mass, which species 
are predominant. Table V, autumn 1910, shows, that Mya truncata is in mass 
quite the dominant form with 240 gm. dry matter per 10 m.? against 282 gm. dry 
matter per 10 m.? for all species. The remaining Lamellibranchs contributed 
ca. 21 gm. Of other animals Ophioglypha is abundant in numbers, in 1910 with 
457 individuals per 10 m.?”, but with only 2.74 gm. dry matter, calcareous matter 
not included; further Pectinaria with 3.70 gm., other worms with 5.72 gm. Of the 
gastropods only Buccinum and Nassa, with together 5.s3 gm., are worth mentioning. 
The content of the Lamellibranchs” stomachs is the finest detritus with 
the relatively few, living, small organisms occurring therein, which only to a small 
extent have anything to do with plankton organisms. 
In 1911 I fitted up a small aquarium with detritus as bottom and then 
had the opportunity of watching how Abra alba obtained the detritus. With the 
one, longer siphon it literally sucks in the uppermost layer of the bottom; the 
detritus can be seen passing through the transparent siphon; after taking several 
helpings, it throws out a part of the stuff, only the very finest passes through the 
animal. It is thus a bottom detritus-feeder. Meyer und Mobius (Fauna der 
Kieler-Bucht 1865—72) also mention that they have seen Abra in the aquarium 
taking in bottom-particles through the one siphon. Other Lamellibranchs, which 
do not have long siphons, must presumably take their detritus from the water 
directly; but these questions require to be more closely investigated in aquaria. It 
