26 MARINE ANIMALS OF MASSACHUSETTS BAY. 



CTENOPHOE^S. 



THE Ctenophorae differ from other Jelly-fishes in their mode of 

 locomotion. All the Discophorous Medusae, as well as Hydroids, 

 move by a rhythmical rise and fall of the disk, contracting and 

 expanding with alternations so regular, that it reminds one of the 

 action of the lungs, and seems at first sight to be a kind of res- 

 piration in which water takes the place of air. The Greeks rec- 

 ognized this peculiar character in their name, for they called 

 them Sea-lungs. Indeed, locomotion, respiration, and circulation 

 are so intimately connected in all these lower animals, that what- 

 ever promotes one of these functions affects the other also, and 

 though the immediate result of the contraction and expansion 

 of the disk seems to be to impel them through the water, yet 

 it is also connected with the introduction of water into the body, 

 which there becomes assimilated with the food in the process of 

 digestion, and is circulated throughout all its parts by means of 

 ramifying tubes. In the Ctenophorae there is no such regular 

 expansion and contraction of the disk ; they are at once dis- 

 tinguished from the Discophoraa by the presence of external 

 locomotive appendages of a very peculiar character. They move 

 by the rapid flapping of countless little oars or paddles, arranged 

 in vertical rows along the surface of the disk, acting indepen 

 dently of each other ; one row, or even one paddle, moving singly, 

 or all of them together, at the will of the animal ; thus ena- 

 bling it to accelerate or slacken its movements, to dart through 

 the water rapidly, or to diminish its speed by partly furling its 

 little sails, or, spreading them slightly, to poise itself with a faint, 

 quivering movement that reminds one of the pause of the hum- 

 ming-bird in the air, something that is neither positive motion, 

 nor actual rest.* 



These locomotive appendages are intimately connected with 

 the circulating tubes, as we shall see when we examine the struc- 



* The flappers of one side are sometimes in full activity, while those of the other 

 side are perfectly quiet or nearly so, thus producing rotatory movements in every 

 direction. 



