l!14 University of California Publications in Zoology. [Vol. (5 



with or without holdfasts, for the presence or absence of the 

 latter has no influence on the result. At the edge of the perisarc 

 the ectoderm changes abruptly by a shortening of epithelial cells 

 and a marked decrease in number of nettling capsules (fig. 12). 

 When the cut is made in this covered portion of the stem, new 

 perisarc is formed within the old, its distal edge proximal to the 

 old edge, a certain proportional distance from the proximal 

 tentacles of the new hydranth. The ectoderm of the region left 

 naked by this shift rapidly acquires the characters that dis- 

 tinguish naked ectoderm on the intact column. 



The position now occupied by the edge of the perisarc is about 

 the same proportional distance from the proximal tentacles as it 

 is in the young hydroid with a single canal, and very different 

 from the usual position occupied in the normal adult. In the 

 course of some days, the normal adult proportions are approxi- 

 mated, by an increase in the length of the stem, involving chiefly 

 the naked portion, and a decrease in the diameter. There is in 

 every regeneration of a hydranth some remodelling of the stem, 

 to form a tapering proboscis and a slender "neck." As the pro- 

 portions of the regenerating piece depart from the normal pro- 

 portions of the species, this remodelling is more and more pro- 

 found. The result in all cases is a more or less successful return 

 to the normal form. This subject will be discussed more fully 

 hereafter. 



A hydranth may develop below a cut made just distal to the 

 edge of the perisarc, so that the distal tentacles and the greater 

 part of the proboscis arise from the naked stem, the proximal 

 tentacles and base of the proboscis from covered stem. The 

 development proceeds as usual. The edge of the perisarc moves 

 hack, uncovering ectoderm which soon becomes indistinguish- 

 able from the original naked ectoderm. One such case is repre- 

 sented in figs. 13, 14. The hydroid has lengthened so as to 

 project from the original perisarc. New perisarc has been 

 secreted, with its anterior limit the usual proportional distance 

 below the proximal tentacles. 



A similar transformation occurs when the regenerating piece 

 has been cut from the naked part of the stem only. A new 

 perisarc is secreted in the proximal region, the ectoderm taking 



