HARDWICKE'S SCIENCE-GOSSIP. 



13 



under the microscope, the green granules (which in 

 the adult Hydra appear a mass of green) could be 

 counted. These had the appearance of beads strung 

 together ; each liad only four tentacles (fig. 13J, 



Fig. ) 3. Young Hydra vMdis, a few days after leaving the ova. 



It now remains to prove in what form the young 

 Hydra first appears on leaving the ova. I do not 

 tliink it has at first the perfect Hydra form, but in 

 all probability is that of a minute grub having no 

 tentacula. In course of time these grow out 



Fig. 15. Detached Ovi-sac. 



I 





14. Parasite of B?/rfca. 



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Fig. 16. Bursting of Ovi-sac. 



gradually, as is the case when they increase by 

 gemmation or budding ; the first form is a lump, 

 which slowly grows out a certain length, and then 

 the tentacles make their appearance, one or two at 

 the first ; and these gradually increase in size and 

 number to the perfect form. Thus those young 

 ones I found at the bottom of the pan had not 

 the exact resemblance in form of the full-grown 

 Hydra, but were uniform in the size of the body, as 

 seen in fig. 14, and with only four very short tentacles, 

 whereas the usual number on the matured IL viridis 

 is from seven to twelve. There is this particular 

 difference between the H. vulgaris and H. viridis — 

 whilst the ova of the former are produced in the 

 autumn] months, and are supposed to sink to the 

 bottom of the pond, and are hatched out in tlie 

 following spring, the latter, on the contrary, 

 produce both [sperm-cells and ovi-sacs at the 

 beginning of summer, and, according to my obser- 



vations, all are hatched out in about fourteen days. 

 From that time up to October tliey have kept 

 increasing by gemmation, and no doubt also from 

 the ova in the vessel I kept them in. On July 9th, 

 1872, I found one more of the H. viridis with the 

 ovi-sac forming as before, but the Hydra was 

 infested with a number of parasites, which ran over 

 every part of the body with a quick motion, and 

 also attached themselves to the ovi-sac after it left 

 the body of the Hydra (fig. 14). I watched with 

 increasing interest the globe-shaped ovi-sac, to see 

 it discharge its contents. I had the good fortune 

 to witness this, and I hoped to have been able to 

 trace the development from the ova (which I believe 

 has never been accomplished yet). But I was 

 doomed to be disappointed, though I saw the ovi- 

 sac burst (as shown in fig. 16). I had the mortifica- 

 tion of seeing these said parasites devour the 

 contents entirely, and that with apparent eagerness. 





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Jor AN tNCH 



Fig. 17. Spermatozoa of Hydra. 



They moved with greater rapidity as they were 

 devouring the minute granules that escaped from 

 it, which, in the course of two hours, had almost 

 entirely disappeared. I saw the spermatozoa issue 

 from the cells both before and after (the ovi-sac, in 

 this instance, was separated from the Hydra's body), 

 and I carefully preserved for two months the water 

 in which it took place, daily looking for a Hydra, 

 if any had escaped the voracity of the parasites, but 

 not one appeared ; still I hope at some future time 

 to be able to trace the development from the ova. 



It has been matter of dispute whether the Hydra 

 has the power of stinging or benumbing its 

 victims ; but this is now generally admitted, and I 

 think there can be no doubt about it. I have seen 

 the small red worm, and also the larva of gnats, die 

 instantly tliey came in contact with the tentacles of 

 the Hydra. The water-flea, Baphnea pidex, on the 

 contrary, will for some short time maintain a struggle 

 with the Hydra : its shell partially protects it from 

 the immediate effects of the poison. Still it seldom 

 disengages itself from the grasp of the Hydra, and, 

 when it has done so, I have seen it in a very short 

 time sink to the bottom of the water and die, 

 evidently from the effects of the stinging. This 

 stinging power is owing to the presence of certain 

 fine threads contained in the numerous tubercles 



