Vol. 3J Torrey. — Biological Studies on Corymorpha. 291 



the vacnolation of the endoderm is not so conspicuous, growth 

 is not so rapid. The suggestion is obvious that there is a causal 

 connection between growth and the absorption of water. In 

 the preceding paper of this series ( :04, p. 408), it was said that 

 the stem may not only shorten without increasing its diameter 

 (indicating a loss of water) but may lengthen while actually 

 increasing its diameter (indicating absorption of water). Re- 

 cently ( :05) it was demonstrated that the axial endoderm could 

 respond locally to local conditions by a change of volume. Tho 

 stem, then, may grow, temporarily at least, by the absorption of 

 water b}^ the axial tissue. It never obtains a permanent length 

 because it is constantly changing in response to stimuli. Its 

 capacity to reach a given length, however, may be taken as a 

 constant, and depends directly upon the number of axial cells 

 and upon their turgidity. 



(c) The Axial Parenchyma as a Skeleton. — Not only does the 

 axial tissue serve as a means by which the length of the stem 

 may be increased. Its presence is correlated with the rudimen- 

 tary character of the perisarc which is delicate, non-supporting 

 and restricted to the proximal third or fourth of the stem. It 

 i? clearly a skeletal tissue, taking the place of the ordinary exter- 

 nal chitinous skeleton of most hydroids. 



3. Division of Hydranth Cavity. — The cushion of vacuolated 

 cells which, in Fig. 27, divides the hydranth cavity into two 

 portions, arises like the axial parenchyma of the stem. Its local 

 development is correlated with the enlargement of the proximal 

 tentacles, whose great length and large size require a reenforce- 

 ment of the hydranth wall at their base;" 



4. The Gonosome. — The first indications of the gonosome ap- 

 pear when there are about twelve proximal tentacles and but h 

 single enteric canal. Two or three protuberances, approxi- 



® May (:03) has published a description and figure of the hydranth 

 of C. pendula which I refer to only because they are so readily accessible 

 and yet are surprisingly inaccurate, misconceived and misleading. The 

 figure is evidently an oblique section (cf. Fig. 27 of this paper). The 

 so-called longitudinal canals, marked b, are but crevices in the parenchyma. 

 There is no sign in the figure of the real base of the hydranth cavity, 

 the fenestrated membrane or the peripheral canals. 



