846 Prof. AUman on the Construction and 



polypoid portion to the vegetative mycelium of a mushroom, and 

 the medusoid portion to the reproductive hymenium, with its 

 protecting parts*. The polypoid and medusoid elements, how- 

 ever, still continued to be treated as primarily independent or- 

 ganisms, receiving each separate generic and specific names j 

 and indeed the time had not yet come when any other plan was 

 practicable ; for the number of free Medusa-forms which had been 

 traced to fixed polypoid forms was far too small to render possible 

 any more philosophic system. 



Observations, however, gradually accumulated, and at length 

 we became aware of a sufficient number of cases in which the 

 connexion between the polypoid and medusoid elements was 

 apparent to justify an attempt at combining the two in our 

 classification. 



Accordingly we find, in an excellent and conscientious paper 

 by M'Cradyt, an attempt made to combine the two elements 

 in his arrangement of the so-called Gymnophthalmic Medusae. 

 M'Crady, however, gives a disproportionate prominence to the 

 medusoid element, and in his nomenclature shows a tendency 

 to adopt a more recent name by which the Medusa may have 

 been known, rather than the older one under which the polypoid 

 element has become familiar to us. 



Agassiz, in the fourth volume of his 'Contributions to the 

 Natural History of the United States,* also shows himself im- 

 pressed with the necessity of combining both elements, in order 

 to allow of our forming an adequate conception of the Hydroid, 

 while he gives no undue prominence to one of these elements 

 over the other, and sees the justice of adopting for the entire 

 Hydroid the name by which it was first systematically described, 

 whether under the form of the free Medusa or of the fixed 

 Polypite-colony. He has thus been frequently compelled to a 

 dismemberment and a redistribution of existing generic groups, 

 as well as to the construction of several new ones. Agassiz has 

 here largely extended our knowledge of the Hydroida, and has 

 made an important advance to a philosophic classification of the 

 group ; but I cannot admit that he has always made a correct 

 estimate of the value of the characters which he employs in the 

 construction of his genera, while he more than once overlooks 

 the just claims of existing names to adoption. 



The necessity of combining the two elements in our concep- 

 tion, description, and classification of the Hydroida is also main- 

 tained in a series of excellent papers published in the ' Natural 



* Dujardin in Ann. des Sc. Nat. 1845. 



t M'Crady, " Gymnophthalmata of Charleston Harbour," in the Pro- 

 ceedings of the Elliott Society of Natural History of Charleston, South 

 Carolina, 1859. 



