472 
The Reproductive Rate of the Bottom Animals 
Petersen, writing of the bottom fauna of the Danish fishing grounds, 
1911—1918, has expressed the opinion that the bottom animals at least 
reproduce themselves in weight each year. This conclusion was based 
by Petersen on the observed fact that from year to year, on the average, 
similar quantities of bottom animals are found over the same areas, along 
with similar populations of bottom-feeding fishes. It is perhaps true that 
more regularity in this respect might be expected in sea-bottom than in 
the bottom muds of our relatively shallow inland rivers and lakes, which 
are subject to extremes of temperature; to floods and consequent wash- 
ing and filling; to disturbance by seining and artificial dredging for navi- 
gation channels; and to other unfavorable influences. 
With more particular reference to growth rates in small bottom 
Mollusca, I note that Petersen (1911) remarks that a small Sphaerium- 
like form, Abra sp., reproduces its weight several times in a year; and 
that F. C. Baker recently observed of very young specimens of the genus 
Ampullaria, from Ceylon, that they doubled in size in an aquarium in 
3 months. 
Body weights taken by us of a series of 3 to 63-month specimens of 
one of the commonest snails from the Illinois River and its connecting 
lakes, Vivipara contectoides, all from late July collections, 1913—1914, 
indicate an average increase in body weight in one year running from 63 
to 100% for the different age-groups studied. 
GROWTH OF VIVIPARA CONTECTOIDES,* HAVANA 
Increased Per cent. 
Age, months Years Maximum Body weight increase 
(estimated ) (estimated) | length of | weight; next next 
shell, mm. mg. 12 months | 12 months 
mg. 
3 ca. 0.25 11.5 160 132 82 
12—15 1.00— 13.0 292 186 63 
24—27 2.00— 15.0 478 438 91 
36—39 3.00— 20.0 916 800 87 
48—51 4.00— 27.5 1,716 1,734 101 
60—63 5.00— 39.0 3,450 | 
The possibilities of multiplication of this particular snail, if left to 
itself, are even better shown by an analysis of a collection from Seebs 
* All weights are of late July specimens, 1913; 3-months’ specimens, evidently 
from spring brood same year; not possible to determine whether specimens one 
year and up were from spring or midsummer broods. 
+ Corrected for loss in alcohol. 
