Reviews and Notices of Boolcs, 61 



as for instance in the numerous radiating tubes that serve it for 

 veins and arteries, and in the little eye specks protected by com- 

 plex lids, that are placed at the margin of the disk, and are its 

 ors'ans of vision. 



The changes whicb these creatures undergo in the progress of 

 their growth, are perhaps more wonderful than any other part of 

 their history. The egg is hatched into a minute animalcule or 

 planula of oval form, and with cilia or spontaneously moving 

 threads on its surface. This fixes itself and becomes a scyphis- 

 toma or hydroid polyp of sack-like form, attached by the base, 

 and stretching forth numerous tentacles from the mouth in search 

 of food. The polyp multiplies by gemmation or budding, so 

 that many may originate from one egg. When full grown it 

 subdivides transversely and becomes a strohila^ which resembles a 

 number of cup-like polyps stuck one into the mouth of another. 

 The strobila eventually breaks up and its separate parts become 

 little freely swimming umbrella-shaped medusae, called epJiyroe^ 

 still quite dissimilar from the parent, to whose form they at 

 length attain in process of growth. The earlier fixed states are 

 passed through during winter, and at the bottom of the sea ; and it 

 is evidently the intention of these remarkable transformations that 

 these creatures shall be capable of quickly filling the summer sea 

 with an abundant brood in each succeeding year, much to the 

 benefit no doubt of multitudes of roving fishes which make the 

 Acalephs their prey. 



The Cyanea thus roughly sketched may be regarded as a ty- 

 pical Acaleph ; but the class embraces a variety of other forms, 

 which are grouped by Agassiz in three orders, (1) Hydroidea, 

 (2) Discophora, (3) Ctenojphora, and it is to the second of these 

 that our Cyanea belongs. 



In the first and lowest of these orders we have the hydra-like 

 polyps, with bodies hollowed into digestive sacks open at top and 

 furnished with prehensile tentacles. Some are solitary and naked, 

 others protected by horny or stony cells, and often grouped by 

 gemmation or budding in complex branching structures, some of 

 them small and resembling sea weeds, and others hard and calca- 

 reous like the true corals ; others again are specialized into sepa- 

 rate tentacular, reproductive and digestive polyps. 



To understand these difi'erences let us suppose an animal re- 

 duced to a mere digestive sack, with or without a horny case or 

 cell, and having tentacles at top, and the power of producing 



