98 



LOWER INVERTEBRATES. 



hydroid are not very great, and although at first sight it might seem as if the two* 

 theories involve very different comparisons, they are in reality identical. 



Order I. — PHYSOPHOR^. 



One of the most interesting forms of Siphonophora is the 

 genus Agalma, the name of which dates back to the days of 

 Eschscholtz, the father of liie study of actinology. It is the 

 type of a family known as the Agalmid^, and belongs to a 

 larger group of Physophorse or float-bearing Siphonophora. 

 The genus Agalma when floating in the water, will be found 

 to be made up of two kinds of bodies. The first of these are 

 transparent, crystalline in apj^earance, and are easily detached 

 from their connections with each other ; the second are more 

 opaque, flexible, and smaller, while they are more tenacious in 

 their attachments to the animal. All are strung together on a 

 common axis or stem which is very flexible in its character. 



The Agalma as it floats in the water is of a very fragile 

 nature. So delicate is it in fact that it cannot be raised out of 

 the water in the hand without the appendages being torn from 

 their connections with each other. The only way to captiu'e it 

 entire is to place under it, as it moves about in the water, some 

 receptacle which will hold liquid, allowing it to float in 

 with the water. The water contained in the receptacle 

 and the animal can then be raised together out of the 

 sea. Even when the greatest care is shown in its 

 capture it retains its appendages but a short time 

 when kept in confinement, and soon loses them 

 all and shrinks to an insignificant size as 

 compared with its former proportions. 

 The axis or stem of the Agalma 

 is a most characteristic structure. 

 It extends from one extremity 

 of the animal to the other, 

 and affords an attachment 

 to all the appendages 

 which make up the 

 whole. It is very 

 flexible, colored a 

 rosy pink, is hollow 

 throughout, and about 

 the diameter of a knit- 

 ting-needle. At one 

 end, which may be 



Fig. 93. — ^ga^ma 6^6(70715/ a, float; 6, nectocalices; c, covei'ing scales; f7, feeding called the upper ex- 

 polyps; e, tentacles and tentacular knobs; f, tasters; g, sexual bells. . ^ 



tremity of the axis, 

 the stem is enlarged into a small globular body which is called the air-bladder or 

 float. This float contains a little sac filled with gas, and in some related genera 



