— 207 — 
something as to the nature of the results obtained by him. Considering these to be of 
great importance to the questions here dealt with, I requested and obtained the favour 
of a brief communication concerning the investigations, and am now, by the courtesy 
of the author, enabled to insert the following particulars as to the variations in oceur- 
rence of bottom organisms in the Limfjord. 
C. G. Joh. Petersen: in the annual numbers of certain 
Invertebrata. 
Fluctuations 
“Dr. Jonan Hsorr has asked me to give some details as to the investigations on 
which I have based my statements*) as to the varying numerical value of certain species 
of lower marine organisms. In accordance with this request, I here furnish the following 
particulars. 
“By means of valuation experiments in the Limfjord, I have at last been able 
to demonstrate the great variation which takes place from year to year among certain 
Invertebrata as regards the number of individuals per m? of the same species. This 
is the only means by which such investigations, based upon actual figures, can be 
carried out. ; 
“As regards the small varieties of common mussels, Abra alba, Corbula gibba, Solen 
pellucidus, the counting can be carried out with great accuracy; 100 or 50 samples were 
taken at different spots embracing the whole of Thisted Bredning, each sample repre- 
senting 0.1 m2. The table below is based throughout upon 50 samples = 5 m?, the 100 
being divided by 2. It was found that 50 samples gave sufficiently accurate results. 
No. of specimens found by valuation in Thisted Bredning'; 50 samples a 0.1 m°. 
a | 
| | | 
| 715], 1909 | 1213/4 1910 | 192/45 1910 | 2/5 1911 | 87/10 1911 | 31751912 | 21101912 | 2141918 | 3191913 
| 
Abra alba........ 32 0 | 419 251 39 | 10 354 302 | 20 
Corbula gibba.... | 2 Be | 1 0 TO || Be 56 452 | 113 
Solen pellucidus. . 146 191 49 78 1131 | 1130 396 515 | 310 
Buccinum unda- | | | 
na PARLES 7 oad | 4 4 EAN are 5 UNSS 
Nissum Bredning; 40 samples 0.1 m°. 
stress ee eee 
ae) 
715]19 1909 19—20},9 1910 | 2/5 1911 | 87/10 1911 | 34151912 | 101912) 213 1913 | 8/9 1918 
; ei f 
Echino- | | | | 
cardium . large 14 22 21 19 SP | | 6 c. 10 
cordatum . | small All probably large. 3 O | 166 240 121 
| 19-19mm |12-25mm.| most; about 
| | 30 mm. 
The large specimens all between 35 and 45 mm. long. 
“For purposes of comparison with the mussels referred to above, the figures obtained 
for Buccinum undatum are also given, this being an organism which in the waters here 
*) In “Valuations of the Sea”, p. 23, English Edition in “Report XXI, Danish Biological 
Station” 1913. 
