208 W. K. BROOKS. 



Now what does this quadrate structure meau ? No one who has 

 watched a pelagic hydro-medusa, as it first swims to the surface, 

 with contracted tentacles, by rapid vigorous pulsations of its bell, 

 and then sinks slowly down, permitting its tentacles to be drawn 

 out by the friction of the water, into long slender threads until they 

 form a living poisoned net, stretched like a spider's web, to entangle 

 the "floating organisms of the ocean ; no one who has witnessed this 

 can doubt the perfect adaptation of its structure to the conditions of 

 its pelagic life. It does not, like a bilateral organism, pursue its 

 prey in horizontal lines, but it captures it while sinking, and then 

 rises to the surface to repeat the process, and its most important 

 relations to space, are radial to the earth and to gravity while those 

 of bilateral animals are concentric with the earth's surface. 



We do not know enough about the biological relations of the 

 medusae to say what physiological superiority definite radiation has 

 over indefinite radiation, but 1 think we may feel confident that the 

 quadrate radiation of the hydro-medusse is an adaptation to a more 

 varied environment than that of a sessil hydra, and that it has 

 been acquired by free animals. In fact the quadrate structure of 

 sessil gonophores is generally accepted as evidence of their descent 

 from free medusse, and the advocates of the hypothesis of polymor- 

 phism attribute the acquisition of the quadrate structure to the free 

 locomotor life of the reproductive persons of the cormus. 



If we admit this what shall we say of the quadrate structure of 

 young hydroids? We might see in the four primary tentacles the 

 accelerated acquisition of medusoid characters if the larval hydroid 

 were not in most cases separated from the medusa by the intervention 

 of numerous generations of hydranths with a great and variable 

 number of tentacles. 



In the actinula of tubularia and in the cunina larva the proboscis 

 or manubrium is long and the body is short, so that the zone where 

 the tentacles are inserted is more nearly equatorial than it is in 

 ordinary hydranths; and the larva, which is ciliated, is thus more 

 perfectly adapted for a pelagic life, 



I shall now show that the quadrate floating hydra and not an 

 ordinary hydranth with an oral crown, is the ancestral type from 

 which hydro-medusse have arisen. 



