154 SUMMARY OP CURRENT RESEARCHES RELATING TO 
other species. Upon the shells of Illyonassa or Littorina the specimens 
are deeply convex and darkly pigmented ; on oyster shells they are much 
flatter and lighter. Dimorphism in C. plana is exhibited by the occur- 
rence of a dwarf form in addition to the normal one. This is a “ physio- 
logical variety ” (a less fortunate phrase) in which the shape and size of 
the body, as well as the number of cells in the entire organism, are 
modified by the direct action of the environment. There is no evidence 
that these modifications are heritable. The dwarfs are not a race, but 
are constantly recruited from the young of the giants. 
Sexual dimorphism is also well marked in C. plana , the average 
female being about fifteen times as large as the average male. The 
smaller size is due to the smaller number of cells in the body. The size 
of the cells is the same. Whatever the ultimate cause of the smaller 
size of the male, it operates, as in the dwarf, by causing a cessation of 
cell growth and division. 
Variations in the Shell of Helix nexnoralis.* — Mr. J. L. Howe has 
made large collections of this snail at Lexington, Va., where the colony 
seems to have been started in 1883 by imports from Europe. His aim 
was to get light on the following questions : — (a) Does the tendency to 
variation proceed along certain definite lines ? ( b ) Does this tendency 
vary in different localities ? and (c) Will a very considerable destruction 
of individuals materially modify the tendency ? His study seems to give 
an affirmative answer to (a) and (b), and a negative answer to (c). The 
colony gives at any rate an interesting example of the results of a 
tendency to variation in a favourable environment, for the snail seems 
to have almost no enemies. 
Habits of Eolis papillosa.f — M. Louis Boutan has made some very 
interesting experiments with this nudibranch which feeds on anemones 
in spite of all their stings. Hecht has maintained that the Eolids which 
have stinging cells themselves are immune to those of hydroids and sea- 
anemones, and has pointed out the interesting fact that the only species 
without nematocysts, Calina glaucoides , feeds on embryos of fishes. But 
the experiments made by Boutan show that the immunity is far from 
absolute ; for a specimen weakened by reproduction was unable to tackle 
a vigorous Anthea cereus, and was engulfed by it. He seeks for the 
source of the relative immunity in the secretion of mucus, which is 
markedly lessened at the reproductive period. 
Functions of the Radula.f — Prof. H. A. Pilsbry makes a note on 
the great length of the radula of Nerita peloronta compared with that of 
the animal, and the large number of similar teeth at the margins. 
Specialisation in the Rhipidoglossa and Pulmonata has proceded from 
the median line of the radula outward, the outer teeth being the last to 
be modified, and therefore of value as indicating the ancestral condition. 
This mode of change is probably the result of the greater functional 
activity of the median portion in feeding, due to the rounded shape of 
the subradular cartilage. 
* Amer. Nat., xxxii. (1898) pp. 913-23. 
f Arch. Zool. Exper., vi. (1898) Notes et Revue, pp. xxxvii.-xlii. 
j Proc. Acad, Nat. Sci. Philadelphia, 1898, p. 202. 
