GENERAL SKETCH OF ACALEPHS. 21 



GENERAL SKETCH OF AOALEPHS. 



IN the whole history of metamorphosis, that wonderful chapter 

 in the life of animals, there is nothing more strange or more in- 

 teresting than the transformations of the Acalephs. First, as 

 little floating planulae or transparent spheres, covered with fine 

 vibratile cilia, by means of which they move with great rapidity, 

 then as communities fixed to the ground and increasing by bud- 

 ding like the corals, or multiplying by self-division, and later as 

 free-swimming Jelly-fishes, many of them pass through phases 

 which have long baffled the investigations of naturalists, and have 

 only recently been understood in their true connection. Great 

 progress has, however, been made during this century in our 

 knowledge of this class. Thanks to the investigations of Sars, Du- 

 jardin, Steenstrup, Yan Beneden, and many others, we now have 

 the key to their true relations, and transient phases of growth, 

 long believed to be the adult condition of distinct animals, are 

 now recognized as parts in a cycle of development belonging to 

 one and the same life. As the class now stands, it includes three 

 orders, highest among which are the CTENOPHORJE, so called on 

 account of their locomotive organs, consisting of minute flappers 

 arranged in vertical comb-like rows ; next to these are the Dis 

 COPHOR.E, with their large gelatinous umbrella-like disks, com- 

 monly called Jelly-fishes, Sun-fishes, or Sea-blubbers, and below 

 these come the HYDROIDS, embracing the most minute and most 

 diversified of all these animals. 



These ' orders are distinguished not only by their striking ex- 

 ternal differences, but by their mode of development also. The 

 Ctenophorae grow from eggs by a direct continuous process of 

 development, without undergoing any striking metamorphosis ; 

 the Discophora3, with some few exceptions, in which they develop 

 like the Ctenophorae from eggs, begin life as a Hydra-like ani- 

 mal, the subsequent self-division of which gives rise, by a singular 

 process, presently to be described, to a number of distinct Jelly- 

 fishes ; the Hydroids include all those Acalephs which either 

 pass the earlier stages of their existence as little shrub-like com- 



