MUSEUM OF COMi'AUATlVE ZOOLOGY. 21 



sis which I offered in a former paper ('91, p. 40) to account for the 

 formation of a septum iu Ectoprocta. 



In favor of this hypothesis are not merely the need of such an appara- 

 tus on account of the frequent loss of the calyx and the lateral branches 

 through accident, and the fitness of this mechanism for the function, 

 but also the existence of the special mechanism of radiate cells, covering 

 over the opening in the dissepiment between the calyx and stalk, — a 

 dissepiment which will be most useful in the manner indicated by this 

 hypothesis, owing to the delicacy of the calyx and its liabiUty to acci- 

 dent. When the lateral branches or the terminal calyx become de- 

 tached from the parent stem, we find that the pore in the septum, 

 remaining behind as a part of the wall of the stalk, has become sealed 

 by a cuticular plug. So also Ehlers ('90, p. 22) in Ascopodaria. In 

 this case we can see the utility of the dissepiment, and can infer its 

 value in those positions where it is not certain, but only possible, that 

 it may be called into play. My conclusion then is, that the dissepi- 

 ments have a purely physiological meaning, possessing a protective 

 function, and that the segments of the stem are only physiological di- 

 visions of a primitively undivided stalk, which have perhaps no other 

 significance than that they are parts separated by the dissepiments.^ 



It follows naturally from the foregoing hypothesis, that the segmen- 

 tation of the stalk has succeeded, rather than preceded, the condition of 

 bud formation from the stalk, it being rendered desirable owing to the 

 greater danger to mutilation to which the stalk is exposed. From this 

 standpoint we can see why buds should be produced on each segment in 

 a similar manner. The relative profuseness of budding in Urnatella is 

 explainable on other grounds. 



Examining more closely the relation of this process to the production 

 of proglottides in a tape-worm, — accepting the view that the production 

 of proglottides is fundamentally a process of continual regeneration of 

 lost parts, — there seems to be an important difference in this, that the 

 growth of the stock of Urnatella is limited, more than ten or twelve seg- 

 ments being rarely formed, while an indefinite number of proglottides 

 are produced. The limited growth of the Urnatella stem seems to indi- 

 cate that the production of segments is not the production of new parts, 



1 Freely branching stocks of Hydroids have septa interpoLated at the base of 

 the hydranth, which is pecuharly Hable to fall off, and sometimes in the middle of 

 the stems. The occurrence of such similar structures throughout the two most 

 profusely branching groups of Metazoa is further evidence for the validity of the 

 physiological explanation of them which I have offered. 



