36 INTRODUCTION TO ZOOLOGY. 



of the outer margin of the disc, the animal striking the water 

 in the opposite direction to that in which it is moving. The 

 motion is easy and graceful, admitting of progress in any 

 direction. The lower surface of the disc is covered with a 

 delicate net-work of vessels, in which the circulating fluids are 

 exposed to the oxygen contained in the sea-water. Each 

 contraction of the margin, therefore, not only impels the 

 animal in its course, hut assists in the process of respiration ; 

 and, as the moving and the "breathing are thus dependent on 

 the performance of the same act, the term pulmonigrades* 

 has been applied to thesje animals; a term no less descriptive 

 than that of u ciliogrades," which, as already mentioned, has 

 been bestowed upon the preceding group. 



The Medusae differ extremely in size. Some are occa- 

 sionally thrown upon our coast which are as large as a good- 

 sized umbrella. While writing these pages, we have before 

 us, in a jar of sea-water, several which are not larger than 

 peas, and some which scarcely exceed in dimensions the 

 head of a large-sized pin. 



Some species are adorned with brilliant colours, and equal 

 in the richness of their hues the brightest of our garden 

 flowers. When, from a small boat, they are beheld rising 

 and falling at pleasure, in a glassy and transparent sea, and 

 occasionally turning over in the apparent exuberance of en- 

 joyment, they are so very attractive as to excite the as- 

 tonishment of the child, while 

 they furnish matter for the 

 contemplation of the na- 

 kturalist, 



Considerable variety pre- 

 vails in the organs for the re- 

 ception and assimilation of the 

 food. In the genus Rhizos- 

 toma (Fig. 20), the arms or 

 peduncles which hang down 

 from the lower surface of the 

 umbrella-shaped disc, are 

 furnished at their extremity 



Fig. 20". HiiizosxoMA. with a multitude of pores. 



By these, the minute animalcules, or the juices of decaying 



PulmO) i\ lung; and (jradior, I walk, or advance. 



