MUSEUM OF COMPARATIVE ZOOLOGY. 19 
2. Segmentation of the Stalk. 
Leidy (’84, p. 9) did not fail to call particular attention to the divis- 
ion of the stalk of Urnatella into segments, and recognized their suc- 
cessive production “ through the process of division, very much in the 
same manner as in the production of the proglottides of tape-worms 
from a scolex.” . 
The phenomena of segmentation must have a special interest on 
account of the relation of segmentation and strobilization. I shall 
treat under this subject, first, of the phenomena of segmentation in 
Urnatella; secondly, of the probable origin and significance of the 
process in this genus; and, thirdly, I shall examine critically Leidy’s 
comparison. 
The transverse septum, which is the first indication of the separation 
of the stalk into segments, begins to be formed at about the time that 
the lateral buds arise, and immediately below them. It arises, as already 
stated (page 8), as a ring-like fold of the ectoderm, whose free edge en- 
closes a circular orifice, through which the interiors of the two segments 
which are being formed are continuous (Plate IV. Fig. 24). By con- 
tinued growth of this fold the diameter of the orifice diminishes, never 
wholly closing, but leaving a space for the passage of the tubular cells, 
to which reference has already been made (page 7). In this process the 
sheath of longitudinal muscles, which originally ran immediately inside 
of the ectoderm continuously from the base of the calyx to the last fully 
formed septum, becomes discontinuous at the place of the ingrowing 
fold, and the separated ends of the muscle fibres become attached to their 
respective faces of the septum. . 
In order to discover the significance of this process of segmentation 
we must first study its distribution throughout the Endoprocta. Usually 
the calyx is borne upon an unsegmented stalk. Such is the condition, 
for example, in Loxosoma, Pedicellina echinata, P. Americana, and 
Ascopodaria. In other cases, Gonopodaria (Ehlers, ’90, p. 146) and 
P. Belgica (Foettinger, ’87, p. 301), there is a poorly expressed and 
highly variable segmented condition. In Arthropodaria (Pedicellina) 
Benedeni the stalk is distinctly and regularly segmented, there being . 
partitions and slight external constrictions. Finally, in Urnatella we 
find the segmented condition still more sharply expressed. 
There is a suggestive parallelism between the formation of segments 
and the production of buds. While with one exception all species with 
unsegmented stalks produce no buds, and none have yet been described 
