THE HYDROIDS. 557 



The whole history of these remarkable animals is curious and interesting in the extreme, 

 for not only do they exhibit some of the most graceful shapes and pleasing hues that can add 

 beauty to a living being, but they also afford examples of the earlier forms of organs and 

 members which in the higher animals attain their fullest development. 



When they have attained their adult condition, they roam the seas freely, though in 

 their earlier stages they are fixed to one spot and assume a shape quite unlike that of 

 their parent. 



The function of nutrition is carried on in these animals in a method sufficiently simple. 

 They are furnished with a cavity, corresponding to the stomach of higher animals, in which 

 the food is placed, and from which a number of diverging vessels convey the nutritive 

 fluid to the rest of the body. 



SIPHONOPHORA. 



For convenience sake, this class is divided into three groups or orders, the first of which 

 is called the Siphonophora, and includes the best organized members of the class. In them the 

 shape of the body is irregular, and there is no central cavity. They are furnished with 

 sucking organs, and move by means of a certain cavity into which water is received gently 

 and from which it is exj>elled forcibly, or sometimes by means of little sacs or vesicles 

 charged with air. 



Owing to the vast number of species contained in this class, it is manifestly impossible to 

 mention all the curious and interesting animals which it includes. Care, however, has been 

 taken to select those species which afford the best types of their orders, and it will be found 

 that almost every group of importance will find its representation in the following pages. 



The present arrangement of the Jelly-fishes (1885), Hydroids so called, and Corals, is 

 under the title Ccelenterata, constituting one of the great branches of the Animal Kingdom, 

 the third in the scale, counting upwards from the lowest. See the classification and nomen- 

 clature tables at end of this volume. Three classes are recognized : Hydrozoa, Actinozoa, and 

 CtenopTiora. In the first and last of these classes are what are familiarly known as Jelly- 

 fishes, while class Actinozoa embraces the Corals and other Sea Anemone forms. 



The first class is Htdkozoa. The first Order embraces those forms called Hydroids. The 

 fresh-water Hydra, is a familiar example. The second Order, Discophora, embraces the 

 great hemispherical jellies that inhabit our North. 



Among the Hydroids. the first class of Ccelenterates, the Titbularia is familiar. It is not 

 uncommon on our Atlantic shores. The plate on page 558 gives a very fine example of 

 this Hydroid. A bunch of these creatures looks more like a group of beautiful pink-like 

 flowers than any other marine form. The color is exquisite pink, while the stems are sober 

 brown. They are found in our North American waters during the summer. The Disco- 

 phores attain the largest size of all. Their popular names are Sea-nettles, in allusion to the 

 stinging powers. Sea-bulbs, etc. 



The bodies of these, though comparatively tough, are yet mostly water. A specimen 

 weighing thirty-four pounds lost ninety per cent, on drying in the sun. These creatures 

 are phosphorescent, glowing like living fire. We have seen the waters of the harbor of 

 Havana one golden hue at night from their presence. The most common form in the 

 Northern waters is the Oyanea, which attains a great size. Mrs. Agassiz records the fol- 

 lowing dimensions from personal measurement, taken from a specimen at Nahant. She 

 says: "Encountering one day one of these huge Jelly-fishes, when out in a row-boat, we 

 attempted to take a rough measurement of its dimensions on the spot. He was lying quietly 

 near the surface, and did not seem in the least disturbed, but allowed the oar, eight feet 

 in length, to be laid along its disc, which proved to be about seven feet in diameter. Backing 

 the boat slowly along the line of the tentacles, which were floating at their utmost extension 



