92 NAKED-EYED MEDUS.E. PART in. 



each, has its own form of thread-cells ; and the stinging 

 power or strength of the poison is nearly in proportion to 

 the size of the animal and the coarseness of its threads. 

 The disk, or umbrella-shaped swimming organ, in both 

 groups consists of a large cavity included between two 

 layers of gelatinous matter, which unite at the rim. 

 The interior membrane, called the sub-umbrella, is en- 

 circled at its edge by a ring of highly contractile mus- 

 cular fibre like the iris of our eyes, by which this swim- 

 ming organ is expanded and contracted. From the 

 centre of the sub-umbrella a stomach, in the form of a 

 proboscis, is suspended, which is of a very different 

 structure in the two groups. 



The Thaumantia pilosella, a member of the naked- 

 eyed group, is like an inverted watch-glass 

 (fig. 112), less than an inch in diameter. 

 The roof of this umbrella is much thicker 

 than the sides, and gradually thins oif 

 towards the rim. The proboscis, or 

 stomach, descends from the centre of the 

 pig. 112. Thaumantia sub-umbrella, but not so far as to the 



pilosella. edge of tte rim . it ends j n a mcm th With 



four sensitive fleshy lips. Four slender canals, which 

 originate in the cavity of the stomach, radiate from the 

 centre of the roof of the umbrella and extend to its mar- 

 gin, where they unite at the quadrants with a canal 

 which encircles the rim, and are prolonged beyond it in 

 the form of tentacles armed with numerous thread-cells 

 containing poisonous darts. These tentacles must be 

 formed of muscular fibre, for they are very irritable : 

 each of them may be extended and contracted sepa- 

 rately or along with the others ; they guide the medusa 

 through the water, and can anchor it by twisting round 

 a fixed object. 



The prey caught is digested in the stomach, the re- 

 fuse is ejected by the mouth, and the nutritious fluid 

 that has been extracted is carried up through the base 



