i6o AN INTRODUCTION TO ZOOLOGY. 



polyp, is upside down : the polyp is fixed, moath 

 upward; the medusa floats, mouth downward, its 

 " Umbrella,'' or dome, corresponding with the fixed 

 end of the polyp. The medusae have the mouth 

 situated in a central hanging disc, called the manu- 

 briura (handle). The gastric cavity, owing to. the 

 shape of the animal, is not elongated, as in the polyp, 

 but spreading. It has branches, which are called the 

 radial canals or gastro- canals. Many medusae 

 possess a nervous system in the form of a ring, 

 and sense-organs, some of these being rudimentary 

 eyes (ocelli), and some rudimentary ears (otocysts). 

 There are three orders of the Hydrozoa: the Hydro- 

 m.ediisce, the Siphonophora, and the Acalepha. The 

 Eydromedusce are colonies of fixed polyps j the sexual 

 polyps are either free medusEe or fixed medusiform buds 

 which never get to the free stage. One of the simplest 

 animals in this order is Hydra. This, however, though 

 in many respects typical, differs in being a single 

 polyp, and not fixed. Like others of the polyps of the 

 Hydromedusee, it has the middle layer of the body 

 very slightly developed, so that the body wall at first 

 sight seems to consist of only two layers, the ectoderm 

 and endoderm. The ova of Hydra are amoeboid cells, 

 somewhat like those of the sponges. It reproduces 

 itself frequently by asexual buds, which do not remain 

 attached for long, but soon become independent. 

 Many of the colonial Hydromedusse secrete horny 

 tubes, the part that surrounds the polyp being called 

 the hydrotheca. The stem has a central cavity con- 

 tinuous with the gastric cavities of the polyps (fig. 35). 



