7^ POTTS : 



Figure 4. An optical section of the same, as shown when 

 looking down through the top of the dome Cor 

 umbrella ). 



(Numbers 3 and 4 were drawn by Dr. J. 

 Percy Moore, of the University of Pennsyl- 

 vania, from specimens that had been preserved 

 for more than nine years in dilute formalin.) 



It will be noticed that in the case of F'igures i and 2 the 

 Medusa' have been named agreeably with the recognized prac- 

 tice in regard to Marine Jelly Fish (possibly because the 

 hydroid forms from which they have been presumptively 

 derived have rarely been definitely determined, while the 

 Medus(z themselves often grow to be conspicuous objects. ) 



My experience with Figure 3 has been different, making 

 me familiar with the almost invisible Microhydj-a for a dozen 

 3'ears before the medusa presented itself. Having thex'efore 

 named the /J^/Z/rr before I saw the fojnidh'>io-, I incline to retain 

 Ins designation. 



The order of development above referred to, and now well 

 known as the Law of the Alternation of Generations, I find 

 described fifty years ago by Philip Henry Gosse. as follows : — 

 " The polyp, a fixed plant-like animal, increases its own indi- 

 vidual life for a while by putting forth a succession of bud- 

 ding heads ; but at a certain period gives birth to a number of 

 beings that bear no resemblance to itself in form or habit, but 

 are, to all intents and purposes, free-swimming medusae. 

 Each of these, after pursuing its giddy course for a time, pro- 

 duces a number of eggs," which, hatching, ultimately produce 

 not meduscc, but, plant-like, shoot up into the hydroids that I 

 am about to describe. "Hence, to use the striking, though 

 homely, illustration of one of the first propounders of this law, 

 " any one individual is not at all like its mother or its daugh- 

 ter, but exactly resembles its orandvwther or its grand- 

 daughter. 



These hydroids then resemble plants, in that they are fixed 

 in position : but instead of feeding almost solely upon iuor- 



