J. FULLAGER ON DEVELorMENT OF Hydrii Vulgaris. 265 



change was observed on January 24th, fifty-five days after extru- 

 sion ; in a few minutes a slight crack was seen in the shell, and a 

 portion of the young Hydra slowly emerged from it, in a rounded 

 form, as shown in fig. 3 F, sketched two hours after the first per- 

 ceptible crack in the 2gg. It continued slowly emerging, and in 

 two hours afterwards rudiments of tentacles appeared, as rounded 

 lumps (fig. 4 G). Seven hours after the first rupture of the ^ggy 

 the tentacles had progressed to the condition shown in fig. 5 H, and 

 in twelve hours the ^z/r^ra was fully developed, with seven tentacles, 

 and in all particulars like the adult, size only excepted. Its 

 appearance was most interesting — delicately pure, and beautifully 

 transparent, as if made of crystal, and still attached to the inside 

 of the shell by the suctorial disc at the posterior end of the 

 body (fig. 6 /). Some specimens finally leave the shell about 

 twelve hours after being fully developed, others twenty-four, or even 

 sixty hours afterwards, when they fix themselves to the bottom or 

 sides of the cell. Their growth is very slow, and I could not dis- 

 cover what they took as food. After a month had elapsed, I intro- 

 duced some small entomostraca, but though they seized them with 

 their tentacles, they could not absorb them. The entomostraca 

 died, however, from the effect of the stinging power of the ten- 

 tacles. The young have not all the same number of tentacles ; one 

 observed had five, most had six, and some few had seven. 



After the extrusion of the ovum, the parent Hydra gradually 

 diminished, the tentacles shortened and slowly disappeared, and in 

 about twenty-one days the whole body dissolved. The sperm cells, 

 three in number, continued on the body for some days after the 

 ovum had been separated from it, and continued to discharge sper- 

 matozoa into the water. 



Both ovisac and sperm-cells are usually found on the same Hydra; 

 but sometimes sperm-cells only are found, when the whole length 

 of the body is studded with them. I have counted in some cases 

 eleven, in others seventeen, and in one as many as twenty -three 

 but where an ovum is formed the sperm-cells rarely exceed four in 

 number. 



The reproduction of Hydra vulgaris from ova takes place in the 

 autumn, and that of Hydra viridis in the spring. 



Description of Plates XII.— XIII. 

 PI. xu. 



Fig. 1. Ovum of Hydra vulgaris, a, transparent envelope, h, short spines 

 c, triangular shape markings on envelope. 



