23 



rytliinic wave-like prof^ression. This condition seems to 

 take the place of the last part of the ordinary and prob- 

 ably more normal free-swimming stage, and is perhaps due to 

 the unfavorable conditions of the laboratory. This condi- 

 tion is not, hovfever, like the pathological plasmodial 

 forms mentioned below. Its changes in shape are slight, and 

 its maTiner of luovement more of a glide than a protoplasmic 

 flowing. None of the definiteness of structure is lost, 

 and these larvae change into hydras as soon as those which 

 transform directly from the free-swimming planulae. It is 

 then not a phenomenon of degeneration, nor, on the other 

 hand, an essential phase in the life of the creature, but 

 rather an intermediate and probably accidental condition. 



V. I-r!i''DRA» - As soon as the planula stage has given place 

 to the settled hydra stage, the coelenteron becomes com- 

 plete. The mouth appears at the tip of the free end, where 

 the tissue has previously showed indications of disintegra- 

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

 cells. At first the mouth is visible only v;hen the speci- 

 mens are killed and cleared or sectioned. Soon, however, 

 it becomes large enough to see in the live animal, by fo- 

 cussing down from above v,'ith a high-power lens; it appears 

 as a minute pit in the ectoderm. The coelenteron is more 



