262 ALTERNATE GENERATIONS. 



there are many others resembling these in all 

 the essential features of their structure, but dif- 

 fering in their mode of development ; for, al- 

 though more or less Polyp-like when first born 

 from the egg, they never become attached, nor 

 do they ever bud or divide, but reach their mature 

 condition without any such striking metamor- 

 phoses as those that characterize the develop- 

 ment of the Hydroid Acalephs. All the Medusae, 

 whether they arise from buds on the Hydroid 

 stock, like the Sarsia, or from transverse division 

 of the Hydroid form, like the Aurelia, or grow 

 directly from the egg to maturity, without paus- 

 ing in the Hydroid phase, like the Campanella, 

 agree in the general division and relation of 

 parts. All have a central cavity, from which 

 arise radiating tubes extending to the margin 

 of the umbrella-like disk, where they unite either 

 in a net-work of meshes or in a single circular 

 tube. But there is a great difference in the 

 oral apparatus ; the elaborate ruffled curtains, 

 that hang from the corners of the mouth, occur 

 only in the Species arising from the transverse 

 division of the Polyp-like young. For this rea- 

 son they are divided into two Orders, the Hy- 

 droids and the Discophorae. 



The third order, the Ctenophora3, are among 

 the most beautiful of the Acalephs. I have spo- 

 ken of the various hues they assume when in 



