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PROCEEDINGS OF THE 



[April, 1857. 



The synonomy of the group stands thus : 

 Crypto carp a?, Eschscholtz. (1829.) 

 Hydroidea, Johnston. (1838.) 

 Gymnopthalmata, Forbes. (1848.) 

 Craspedota, Gegenbaur. (1856.) 



Of these, the first in order of time, that of Eschscholtz, ex- 

 pressing an erroneous view as to the economy of these animals, 

 has been by common consent abandoned. The Gymnopthalmata 

 of Forbes, and the Craspedota of Gegenbaur, besides being based 

 upon characters which do not characterize all the genera in the 

 group, are subsequent in time to the Hydroidea of Johnston. Two 

 years ago Prof. Agassiz informed me of his intention to include 

 the Hydroid Polyps, the Naked-eyed Medusae, and the Siphono- 

 phorae in a single order under the name Hydroidea. This appears 

 to me the only solution of the question as to nomenclature. 



Class ACALEPHiE. 



Order Hydroidea Agassiz. 



Syn. Gymnopthalmata j Forbes. 

 Craspedota. Gegenbaur. 

 Hydroidea, Johnston. 



This Order includes the lowest of the Acalephae, and the animals 

 which compose it are distinguished by the following characters: 



The general form of the body varies within a wide range. It is 

 sometimes acorn-shaped or conical, often mushroom-like, or with 

 the form of an umbrella, in many forms lenticular, but most fre- 

 quently distinctly campanulate. There are many compound forms 

 where the Medusas, developed by gemmation from their larvae, re- 

 main organically connected with them during life, and thus form 

 floating or fixed plant-like communities. The animals therefore 

 are either fixed or free. When fixed, though possessing sexual 

 organs, their other parts are frequently rudimentary, or even en- 

 tirely wanting. Or the disk may be well developed while the 

 digestive, sexual and tentacular systems are wanting, or the whole 

 animal may have the form of a tentaculum, or of a leaflet pierced 

 by tubes. Or again, the digestive trunk may be the only part 

 developed. 



The Hydroidea are distinguished by a simple digestive cavity, 

 of various forms from that of a depressed chamber, to thai of a 

 very much elongated tube. The circulatory system consists of 



