OF THE HYDROMEDUSiE. 363 



with the rhythmical contractions of Medusae. It was quite plain from this, that ex- 

 pulsion had taken place, but still- J had not seen the expelled animals until some time 

 after the occurrence, and it was not until a later date, Sept. 18th, that I had an oppor- 

 tunity of observing the condition of the larva at the time of expulsion. From this I 

 learned that shortly after assuming independence the larva changes the Medusa-form, 

 under which it is first freed, for another which is more persistent." 



Although McCrady believed, at the time his first paper was written, that the larvae 

 were the young of Turritopsis, he discovered that their position inside the bell is not 

 their primitive one, and the youngest larvae which he found, and of which he gives the 

 following description, were on the bell-margin among the tentacles. He says of this 

 stage, "It was proboscidian and apparently unprovided with tentacles. * * * It 

 was clinging to the tentaculiferous border of the parents' disk, by means of the extrem- 

 ity of its own proboscis. This circumstance also was peculiar, since in no other instance 

 have I seen the larva to use the proboscis as even a means of temporary adherence for 

 the purpose of locomotion. Its position, also, at the border of the disk, is worthy of 

 especial notice, for the habitual position of the tentaculated larva, is on the sides of the 

 proboscis of the parent, or clinging to the inner surface of the upper part of the swim- 

 bell, and in no other instance have I been able to satisfy myself that there was any 

 adhesion to the tentaculiferous border. Just within the cavity, and almost on the border 

 of the veil, it clung with such tenacity that, notwithstanding the powerful contractions 

 of the parent, by which it would be thrown, now within and now without the opening 

 of the swim-bell, its hold was never lost. Yet it appeared to be in contracted con- 

 dition from the constant irritation to which, by its position, it was subjected. Prom the 

 same cause I was prevented from making anything but an outline." This rough outline 

 seems to show that this larva was like, or perhaps a little younger than, the one shown 

 in PI. 43, fig. 1. 



This also was found on the bell-margin, and consisted of a body with two short 

 stout tentacles, ending in rounded batteries of lasso cells, and a very long proboscis 

 with a very small mouth. The digestive cavity, c, is lined by large ciliated endoderm 

 cells,/, and the mouth may be closed until it is almost invisible, or it may be widely 

 opened. The ectoderm, which contains scattered lasso cells and spots of brown pig- 

 ment, is thin everywhere, except at the tips of the tentacles and at the aboral end, be- 

 tween the bases of the tentacles, where it forms a thickened pad, g. 



When detached it swims or glides slowly through the water, and as floating particles 

 are driven away from the surface, there can be no doubt that the ectoderm is covered 

 with small cilia, although I was not able to see them. McCrady says that his specimens 

 had no mouth or tentacles, but his figures show that the larva was essentially like the 

 one which I have drawn, although the tentacles may possibly have been a little shorter. 

 The very close similarity between this larva and the larva of Polyxenia which 

 Metschnikoff has studied (51), renders it probable that the egg in this case also gives 

 rise to a ciliated planula which iloats in the water, and acquires a stomach and a mouth, 

 and that two opposite tentacles are then developed, either just before or just after it fixes 

 itself to the Turritopsis. The rhizopod-like stage, which Metschnikoff (52) has de- 

 scribed in the parasitic Cunina-larva which is found on Carmarina, is probably absent in 



