72 ZOOLOGY: G. H. PARKER 
the theory that the South Atlantic bridge was present even in the 
Miocene. 
^Gage], C, Handhuch der Regionalen Geologie, 7, Pt. 10, Heidelberg, 1910. 
2 Barrel!, J., Amer. J. ScL, New Haven, Conn., (Ser. 4), 40, 1915, (1-22). Also see Wright, 
W. B., The Quaternary Ice Age, London, 1914, chaps. 16 and 18; and Goldthwait, J. W., 
Amer. J. ScL, New Haven, Conn., (Ser. 4), 32, 1911, (291-317). 
aPerret, F. A., Amer. J. Sci., New Haven, Conn., (Ser. 4), 42, 1916, (443-463). 
THE RESPONSES OF HYDROIDS TO GRAVITY 
By G. H. Parker 
ZOOLOGICAL LABORATORY. MUSEUM OF COMPARATIVE ZOOLOGY. HARVARD COLLEGE^ 
Read before the Academy. November 14, 1916 
Corymorpha palma is a solitary hydroid occurring on the mud-flats 
of False Bay and other like localities in the neighborhood of La Jolla, 
southern California. It has a length of body that may exceed even 6 
or 7 cm. In its natural position under water its basal end is imbedded 
in the nfud, above which its stem rises vertically, carrying at the opposite 
end the somewhat drooping head. When Corymorpha is removed 
from the mud and allowed to attach itself to some foreign base which 
can be conveniently turned in an aquarium, it assumes in a very 
short time a vertical attitude irrespective of the position of the base. 
This vertical attitude is acquired and maintained by the stem even 
after the head has been cut off and this part may therefore be said to 
exhibit negative geotropism. The stem is provided with a neuromus- 
cular sheath and a core of vacuolated cells like those in the chorda of 
vertebrates. Torrey has raised the question of the relative importance 
of the neuromuscular sheath and of the core cells in bringing about the 
geotropic response and has advanced evidence in favor of the view 
that the core cells, acting as certain vegetable tissues often do, are the 
mechanism of this response. If, however, Corymorpha is placed in 
sea-water containing some chloretone, by which the neuromuscular 
activity is abohshed but the core cells are left unchanged, no geotropic 
response can be obtained when the animal is moved out of the vertical. 
When the core cells are disorganized by twirling a needle in the axis 
of the stem, care being taken that the neuromuscular sheath is not in- 
jured, the stem will show a tardy but successful geotropism. It there- 
fore seems probable that the geotropic response in Corymorpha, as in 
most other animals, is the result of the activity of the neuromuscular 
sheath and not of the core cells, though the latter very probably help 
to maintain the geotropic position by assuming a somewhat fixed 
