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PRESIDENT S ADDRESS. 
instance in docks at London, but has succeeded in adapting itself 
completely to a fresh-water habitat. 
Cordylopliora has formed the subject of a recent note by Pauly,* 
who calls attention to the modifications in structure which have 
been connected with its migration from the brackish water at the 
mouth of the River Warnow, near Rostock, into the completely 
fresh water of the upper reaches of the river. The fresh-water 
specimens are much smaller f than those found a few miles off in 
brackish water ; they branch much less freely ; and gonophores 
are produced in smaller numbers. The change to fresh water has, 
however, its compensating advantages, since the number of polypes 
is much larger in specimens found there than in those which occur 
in the brackish water. This difference is not to be ascribed to the 
direct action of the environment, but it finds a much simpler 
explanation as the result of the observation that the brackish water 
forms are exposed in the summer to the attacks of a small 
Nudibranch Mollusc, Aeolis exiyua, which systematically eats off 
all the parts of the colony which are not protected by perisarc. 
The Mollusc is unable to follow the Hydroid into fresh water. 
The facts connected with the distribution of Cordylopliora in the 
Bure district are well worthy of further observation. 
The need of work on fresh-water animals is well illustrated by 
a paper just published by Lauterborn,} with a view of calling 
attention to a special fauna and flora found in the Rhine district, 
in small ponds covered with Duckweed ( Lemna trisulca). The 
bottom of a pond of this kind is covered by mud containing 
* Zoolog. Anzeiger, vol. xxiii., 1900, p. 546. 
t The measurements given are 0 - 8 — To cm. for specimens from fresh 
water, and 3 — 8 cm. for those from brackish water. There is a considerable 
discrepancy between these dimensions and the size of a large colony, 8 inches 
in length ( = 20 cm.), recorded by one of the members of this Society, 
Mr. Scherren, from the liiver Thurne, towards Hickling, attached to 
a Potamogeton (‘ Nature,’ vol. xliv., 1891, p. 445). The measurements given 
by Pauly correspond more closely with those of a form from the River Elbe, 
described by Kirchenpauer as C. albicola (see Allman, l. cit. p. 254). 
X Lauterborn, “Die ‘sapropelische’ Lebewelt,” Zoolog. Anzeiger, vol.xxiv. 
1901, p. 50. 
