SECT. in. TUBULARIID^E. 91 



The most common form of the family of the Tubu- 

 laria has no branches : it has an erect, hollow stem 

 like a straw, sometimes a foot high, coated by a horny 

 sheath. The polype which terminates each plant has 

 a mouth surrounded by alternately long and short ten- 

 tacles. The stomach of the polype is connected with 

 the hollow in the stem by a muscular ring, by whose 

 alternate dilatation and contraction, at intervals of 

 eighty seconds, the fluid is forced up from below, enters 

 the stomach, and is again expelled. Another liquid 

 carrying solid particles circulates in a spiral through 

 the whole length of the stem. Some of this family are 

 propagated by perfect deciduous medusae, others by im- 

 perfect fixed ones; both are developed on the polypes 

 or among their tentacles. Like the fresh-water Hydras, 

 these creatures can restore any part of their bodies that 

 is injured. 



Numerous instances might be given to show that the 

 minute medusiform zooids are only a stage or phase in 

 the life of an oceanic hydra : conversely it will now be 

 shown, that the single simple hydra is but a stage in the ^ 

 life-history of the highly organized medusa, jelly-fish, \ 

 or sea-nettle of sailors, the Acalepha of Cuvier. 



The medusas vary in size, from microscopic specks 

 that swim on the surface of the sea in a warm summer 

 day to large umbrella-shaped jelly fish almost a yard in 

 diameter. They abound in every part of the ocean and 

 in all seas, often in such shoals that the surface of the 

 water is like a sheet of jelly. Their substance is trans- 

 parent, pure, and nearly colourless ; chiefly consisting of , 

 water, with so little solid matter, that a newly caught 5 

 medusa, weighing two pounds, dries into a film scarcely i 

 weighing thirty grains. 



The Pulmograde Medusas, which swim by the con- 

 tractions of their umbrella-shaped respiratory disc, 

 form two distinct groups, the naked-eyed medusas and 

 the covered-eyed group. Both are male and female; 



