1902.] NATURAL SCIENCES OF PHILADELPHIA. 761 



varying time. Toward the end of this time the first indications of 

 a coelenteric cavity appear in the arrangement ot the cells at the 

 posterior end of the swimming larva (PI. XXXI, fig. 11, P). 

 Their inner margins come to lie in a straight line, following the 

 long asis of the larva (PI, XXXI, fig, 11, C). This process is 

 better understood when we notice that in changing its shape from 

 the spherical morula to the elongated planula the larva also under- 

 went a slight rearrangement of its cells. The endoderm was first 

 formed as a spherical mass, and its cells were all conical, radiating 

 from the centre to the surface. But as elongation took place in 

 the formation of the planula, the cells were stretched out into a 

 cylinder and their inner ends overlapped irregularly, as is shown 

 at the anterior end (A) of fig. 11. When the coeleuteron begins 

 fo be developed, the inner ends of these upper endodermal cells 

 change their position somewhat and, as above stated, meet along a 

 continuous line. At the same time a change is to be noticed in the 

 cells at the surface of the oral pole. The cell walls at this point 

 become less distinct (PI. XXXI, fig. 11, 0), and finally a disin- 

 tegration of the boundaries leaves the tissue an undifferentiated 

 layer of protoplasm. Before separation of the tissue to form the 

 definitive coelenteric cavity, the larva stops swimming, loses its cilia, 

 and settles down upon the bottom. The larger end, which was 

 directed forward in swimming, is downward. Between the free- 

 swimming stage and the sessile hydra-stage there frequently, 

 though not always, intervenes a condition which reminds one of a 

 minute planarian in its shape and movements. The planula settles 

 down upon the bottom and slowly glides along by a rhythmic wave- 

 like progression. This condition seems to take the place of Ihe last 

 part of the ordinary and evidently more normal free-swimming 

 stage, and is perhaps due to the unfavorable conditions of the 

 laboratory. This condition is not at all like the pathological 

 plasmodial forms to be mentioned below. Its changes in shape are 

 slight, and i(s manner of movement rather a glide than a proto- 

 plasmic flowing J[one of the definiteness of structure is lost, and 

 these larvae transform into hydras as soon as those which change 

 directly from the free-swimming planulse. It is, then, not a 

 phenomenon of degeneration, nor, on the other hand, an essential 

 phase in the life of the animal, but rather an intermediate and 

 probably accidental condition. 



