202 W. K. BROOKS. 



It may be that the pelagic habit of the acraspeda is secondary, 

 and while the polyps, the craspedota and the acraspeda have un- 

 questionably had a common starting point in a free pelagic ancestor, 

 it is quite possible that the more immediate ancestors of the acras- 

 peda were fixed polyp-like inhabitants of the bottom. 



As our opinion of the origin of the medusse has been evolved 

 under the erroneous notion that the craspedota and acraspeda are 

 very closely related and have travelled the same path in company, 

 it has been assumed that, so far as remote ancestry is concerned, 

 what is true of some medusse must be true of all. 



If the remote ancestors of the acraspeda were inhabitants of the 

 bottom, and if their pelagic habit is a secondary acquisition, it is 

 natural to assume that this must be true of the craspedota as well, 

 although all authorities now agree that the swimming habit has 

 been independently acquired in the two groups, and that the acras- 

 peda teach us nothing of the phylogeny of the craspedota. 



Of the four grand divisions of the craspedota, two, the narco- 

 medusse and trachomedusas are inhabitants of the open sea, seldom 

 found by the shore collector, and they are so active and irritable 

 and so easily destroyed that general collections of pelagic animals 

 contain few traces of their existence. 



While the geryonids are not uncommon near the shore, it is only 

 by oceanic collecting and by research at sea that we can form a just 

 conception of the abundance and diversity and scientific importance 

 of the trachomedusse and narcomedusse ; and thus the impression 

 has arisen that they are aberrant and exceptional, and that the more 

 familiar leptomedusse and anthomedusse and hydroids are the 

 characteristic and typical members of the group. 



The hydroid-corraus is a most conspicuous and impressive feature 

 in the life of all our common hydro-medusse, and in many of them, 

 as in eudendriura, it is everything, and the medusoid structure is so 

 degenerated that its existence is a matter of philosophy, rather than 

 an observed fact. 



All these factors have combined, during the historical growth 

 of zoology, to give to the hydroid cormus a fictitious value, and 

 this result has been promoted by the fact that many specialists have 

 devoted themselves to the systematic study of the hydra-stage alone, 

 to the neglect of the medusa-stage. 



