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THE POPULAR SCIENCE MONTHLY. 



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hularia, and the little fruit-like clusters are persistent jelly-fishes which 

 develop and remain just beneath the row of tentacles (Fig. 13), instead 

 of becoming free jelly-fishes as in Sarsia and in Campanularia. 



In the Gulf of Mexico are communities of hydroids so organized 

 that they seem to constitute but one animal. 

 Such is the well-known " Portugese man-of- 

 war " (Fig. 15). This community consists 

 of an elegantly-crested air-sac floating upon 

 the water, and giving off numerous long and 

 variously-constructed appendages. Accord- 

 ing to Agassiz, the different parts are so 

 many different kinds of members in the com- 

 munity, and fulfill widely-different offices, 

 some catching and eating food for the whole, 

 others producing buds, others being the 

 locomotive or swimming members, and hav- 

 ing tentacles that in some cases are twenty 

 or thirty feet long. The air-sac itself is only 

 a few inches in length. 



But the most common jelly-fishes are 

 those which are more or less disk-shaped, 

 and hence are called the Discoplioroe. The 

 " sunfish " is one of these. This name, we 

 hasten to say, is rather indefinite when 

 used without modification, for it is not 

 only applied to a jelly-fish, but it is also 

 given to our fresh- water bream, and to one 

 of the large marine fishes — orthogoriscus. 

 The " sunfish " of which we now speak 

 attains a diameter of six to twelve inches 

 (Fig. 19). In the early spring it may be 

 seen in large schools near the surface of 

 the water, and at this time is only a small fraction of an inch in diam- 

 eter. It becomes full-grown by the middle of summer, and great num- 

 bers may then be seen swimming slowly by a sort of motion that may 

 be likened to that of partly shutting and opening an umbrella. The 

 motion is indeed effected by the contraction and expansion of the 

 whole umbrella-like disk. 



In the study of the sunfish {Aurelia) we are able to see plainly 

 the prominent differences between jelly-fishes as a group and polyps 

 as a group. 



The natural attitude of the latter is with the mouth upward, or at 

 least not turned downward, and the body is divided into vertical 

 chambers, by vertical partitions, and the substance of the animal is 

 flesh-like. On the other hand, the typical adult jelly-fishes have their 

 mouth on the under surface, or at least not turned upward, their sub- 



FiG. 15.— Portuguese Man-of- 

 War {Physalia arethusci). 



