UNIVERSITY OF CALIFORNIA PUBLICATIONS 
ZOOLOGY 
Vol. 2, No. 6, pp. 323-332, Figs. 1-4 December 13, 1905 
CONTRIBUTIONS FROM THE LABORATORY 
OF THE 
MARINE BIOLOGICAL ASSOCIATION OF SAN DIEGO. 
Vb 
DIFFERENTIATION IN HYDROID COLONIES 
AND THE PROBLEM OF SENESCENCE. 
BY 
HARRY BEAL TORREY. 
An examination of Pacific Coast hydroids during the past 
few years (vid. Torrey, :02, :04) has convinced me that many 
if not all species change their structural type with age. Stems 
straight proximally may become sinuous distally. Branches 
which alternate during the early stages of colonial development 
may later originate in pairs. Length and annulation of hydro- 
theeal pedicels, size, proportions and.ornamentation of hydro- 
theeae may similarly vary with the distance from the base of 
stem or branch. One hydroid in particular exhibited marked 
changes in structure with the growth of the colony and appeared 
to be especially favorable material for the investigation of cer- 
tain questions of differentiation. 
This species was described some months ago as Clytia bakeri' 
(Torrey, :04, p. 16). The stems, usually unbranched, rise from 
a ereeping hydrorhiza to the height of 20 or 30 mm. For a 
few millimeters (2 to 5) each stem is free of polyps and is ecom- 
pletely annulated. The annuli, short at the base, increase grad- 
ually in length until they become, at the first hydranth pedicel, 
1The Hydroids of the San Diego Region. Univ. Cal. Publ. Zoology, I, 
No. 1, 1904. 
