312 PROCEEDINGS OF THE ACADEMY OF [1887. 



Rosel ill 1755, represents H. vlrldls in the same condition and with 

 the arms in the same proportionately short state (Insecten Belustig- 

 ung, Theil 3, Tab. 88, Fig. 4.) In other characters, the speaker 

 found our green Hydra to accord with H. vlridls ; and further in 

 respect to the sexual organs. Prof. Allen Thompson describes the 

 latter as producing a single ovary near the middle of the body and 

 two or three spermaries from the body just below the arms (Edin- 

 burgh Philos. Jour. 1847, 281.) The same condition he had ob- 

 served in our green hydra, as represented in the drawings exliibited. 

 As regards our brown hydra, Agassiz gives as the distinctive char- 

 acter, that it has very short arms while the European has long ones. 

 Ordinarily this appears to be the case, but on several occasions the 

 speaker had observed our brown hydra, after it had been kept some 

 time in an aquarium where there was comparatively little food, 

 elongate its arms, extremely attenuated, even to a length of three 

 inches. In this condition it closely resembled in appearance the 

 beautiful figure of H. fusca in figure 1, plate 64. of the Regne Ani- 

 mal of Cuvier. 



He had the opportunity of seeing both the green and brown hy- 

 dra west of the Rocky Mts. and these he found to accord in character 

 with our eastern forms. In specimens collected in a lake in the 

 Uinta Mts., Wyoming T., at 10.000 feet elevation, the brown hy- 

 dra at first was brick red with a brighter red head, but after keep- 

 ing it for a week, it assumed the pale brown hue as ordinarily ob- 

 served in the animal nearer home. 



The characters of the two American forms as observed by him 

 are as follows : 



Hydra viridis ? The green hydra. Animal bright grass green 

 sometimes paler. Body when moderately elongated cylindro-conical, 

 tapering towards the caudal end ; when contracted oval or spheroid , 

 when greatly extended linear cylindrical. Head conical. Arms 

 four to seven, commonly six, about half the length of the body, lin- 

 eal', capable of extension to about the length of the body or slightly 

 more. In the sexually mature state : testes hemispherical sur- 

 mounted by a nipple-shaped prominence, situated on the sides of the 

 body just below the arms ; ovary single, projecting from near the 

 middle of the body and containing a single, spherical, white egg, en- 

 closed in a brownish covering. Animal usuall}^ three or four lines 

 long, capable of extension to twice the length or contracting to less 

 than a line. In ponds and ditches in the vicinity of Philadelphia 

 and other ])laces, though not common. Observed on one occasion 

 in tlie sexually mature condition late in autumn. In the in- 

 dividuals observed the sexes were separate ; the males with the two 

 testes, and the females with a single ovary. The ovum measured 

 0*375mm in diameter. In the sexually mature H. viridis observed 

 by Prof A. Thompson, individuals were hemaphrodite while in 

 others the sexes were separate. 



