762 pkoceedings of the academy of [dec, 



The Hydra. 



As soon as the planula-stage has given place to the settled hydra- 

 stage the ccelenteron becomes complete. The mouth appears at the 

 free end where the tissue has previously showed indications of dis- 

 integration, at the end of the axial line formed by the endodermal 

 cells. At first the mouth is visible only when the specimens are 

 killed and cleared or sectioned. Soon, however, it becomes large 

 enough to see in the live animal by focussing down from above with 

 a high-power lens. It then appears as a minute pit in the ecto- 

 derm. The coelenteron is more distinct at the upper end than 

 below, where it disappears into the loosely constituted cell-mass of 

 the interior. The definite cavity of the coelenteron is somewhat 

 later in making its appearance. When finally established it is 

 lined with a thick layer of columnar endodermal epithelium. At 

 its bottom it flares out in following the contour of the body-wall, 

 as it appears in PI. XXXII, fig. 15, which shows a late stage, 

 but the same condition of the coelenteron as exists in the newly 

 transformed larva. The figure also shows a thickened core of 

 endoderm which projects upward into the coelenteron as a gastric 

 peduncle. This conical mass of cells develops during the latter 

 part of the hydra-stage. 



A. Tentacles. — In the later transformation of the developing 

 Gonionema no definitely determinate periods separate the times of 

 active change. The development time is variable, depending upon 

 external conditions of food, temperature, etc. In an average 

 larva, however, the first tentacles make their appearance during the 

 third week after the fertilization of the egg, or a week after the 

 larva becomes attached. Two tentacles appear opposite one another 

 at a level about one-quarter of the distance from the upper pole of 

 the hydra (PI. XXXII, fig. 12). They are knob-like when they 

 appear, but grow rapidly to a considei'able length, the few endo- 

 dermal cells which form the core of each tentacle increasing in 

 number. Fig. 12 shows a vertical section of a two-tentacled polyp 

 of the fifth week. The manner of origin of the tentacles will be 

 described in the section on the origin of tentacles under "The 

 Medusa. ' ' 



The second pair of tentacles (PI. XXXII, fig. 14) appear soon 

 after the first, and by their rapid growth soon become as large as 

 the first pair, from which they are then no longer distinguishable. 



