MUSEUM OF COMPAKATIVK ZOOLOCIY. 19 



2. Segmentation of the Stalk. 



Leidy ('84, p. 9) did not fail to call particular attention to the divis- 

 ion of tlie stalk of Urnatella into segments, and recognized tlieir suc- 

 cessive production " tlu-ougli 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 tliis 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. 



Tlie 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, thi'ough 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 



