ACALEPHA. 39 



But besides these common forms, which include 

 nine-tenths of our native representatives of the 

 Class, we have a few species of very different ap- 

 pearance. The most familiar of these are the 

 Beroes and Cydippes, which look like tiny melons 

 of glass, down whose bodies run bands or meridian- 

 lines of paddles, which are the organs of locomo- 

 tion. In the kinds above described, swimming is 

 performed by alternate contractions and expansions 

 of the whole disk ; but in these it is effected by 

 cilia set in short transverse rows, many of these 

 rows making one meridian-band ; these cilia-rows, 

 moving up and down, strike the water with the 

 most beautiful regularity, exactly like the paddles 

 of a steamer, and row the little crystal globe along, 

 with an even, graceful, gliding motion. 



Others again, as the Portuguese man-of-war 

 (Physalia) , are floated by a large bladder filled with 

 air, and are driven along the surface of the sea by 

 the winds. Others, again, have their jelly-like 

 flesh stretched over a plate of cartilage, as the 

 Sallee-man ( Velella), with a number of short ten- 

 tacles dependent from the under-surface, while a di- 

 agonal plate stands up perpendicularly above, and 

 acts as a sail. These kinds, however ,*are only acci- 

 dental stragglers to our shores from warmer seas. 



A very interesting point of connexion between 

 this Class of animals and the preceding is the 

 interchange of form. Some of the ZooPHYTA, as 

 the Tubulariadoe and the Campanulariadce, give 

 birth to a progeny which are, in every respect, 

 Naked-eyed Medusse ; while, on the other hand, 

 the young of the Medusse are, in their earlier 

 stages, stationary Polypes.* 



* For many details of this "Alternation of Generations," 



