Vol. 3] Torrey. — Biological Studies on Corymorpha. 279 



pronounced that the term metamorphosis may well be applied 

 to it. The single central cavity of the larval stem is rapidly 

 reduced to a series of small peripheral canals by the establish- 

 ment of a central rod of immense, vacuolated skeletal cells. The 

 stem, at the beginning about as long as the hydranth, increases 

 in length until it may become ten times as long as the latter, 

 whose cavity meanwhile is divided by a large cushion of .vacuo- 

 lated endoderm cells into pre- and post-tentacular portions. 

 Finally the perisarc is pushed back until it covers no more than 

 the proximal third or fourth of the stem. In structure and pro- 

 portions, then, larva and adult are strikingly different, a fact 

 to which we shall return in the later consideration of the prob- 

 lems of regenerative development. 



1. Recession of the Perisarc. — The external changes may bt 

 considered first. The beginning of the annular ectodermal 

 thickening which finally marks the oral limit of the perisarc is 

 shown in Fig. 11, being the result of an increase in length of 

 the cells in its course. It is here close behind the constriction 

 which marks externally the aboral limit of the hydranth. As 

 yet the perisarc envelopes the whole individual. In Fig. 26, an 

 optical section of a polyp with four distal and six proximal 

 tentacles, the thickening has broadened and shifted backward a 

 distance nearly one-half the length of the hydranth. It is now 

 the oral boundary of the perisarc. The section represented by 

 Fig. 27 is through a polyp with live or six peripheral canals. It 

 shows that the thickening, which maintains about the same rela- 

 tive position, has changed its character. Fig. 28 is a detail of 

 the zone of transition between the ectoderm above and below the 

 perisarc, showing that the thickening is still due to the elonga- 

 tion of ectoderm cells. The naked ectoderm exhibits a new 

 tendency to roll over the edge of the perisarc ; this becomes more 

 pronounced Avith age. The cells beneath the perisarc stain more 

 deeply and wear the aspect of gland cells. In both naked and 

 covered ectoderm, nematocysts develop, but mucli more abund- 

 antly in the former. 



From this time the perisarc recedes gradually to its adult 

 position. The change is due partly to the general elongation of 



