14 



HARDWICKE'S SCIENCE-GOSSIP. 



•with which the tentacles are thickly beset, called 

 urticating threads, which the animal has the power 

 of pushing out at its pleasure, thereby rendenng 

 the surface of the tentacle rough, and giving a 

 firmer hold on its living prey. I have, by pressing 

 a Hydra between two pieces of glass, forced out 



iQji 





^^mmmwm 



Fig. 18. StiiigingOr- 

 gan of Hydra vul- 

 gin-is when pressed 

 out of vesicle. 



^^ 



Fig. 19. Arrow-headed stings projected 

 out of vesicle : ti, recurved hooklets 

 expanded ; 4, ditto at rest. 



these urticating threads from the vesicles, together 

 with the three recurved hooklets attached, by which 

 the tentacle is rendered so powerful an instrument 

 of retention, and have them dried out and mounted 

 for the microscope (fig. 18). The hooklets are clearly 

 and distinctly seen when first pressed out and before 

 they are dried, but are not so plainly seen after, as 

 they close down iu their natural position of rest. 



In fig. 19 is given a rough sketch of the way in 

 which the arrow-headed sting is projected out of the 

 vesicle in connection with the urticating thread, 

 which gives the power to the Hydra of holding the 

 struggling victim. I have often wondered at the 

 tenacity with which the Hydras maintain their hold 

 on a water-flea, when perhaps only the extreme end 

 of the polyp's tentacle at first has come in contact 

 with the very fine antennce of the flea, and the flea 

 has endeavoured to escape by its characteristic 

 violent jerks, and I thought them to be powerful 

 enough almost to break the tentacle itself. But the 

 tentacle is thickly set with these arrow-lieaded 

 stings (fig. 19, a), so that when the animal puts forth 



a number of them it is easy to see how difficult it 

 would be for its prey to escape its grasp. These 

 threads must of necessity be composed of something 

 very different from the other parts of the Hydra, 

 for they continue visible under the microscope 

 after the other parts of the body are dried up or 

 dissolved. 



The parasite on the Hydra was observed more 

 than a century ago by Trembley, who made the 

 Hydra his special study. He calls them lice, and 

 observes that they sometimes kill the Hydra ; and 

 he gives some curious instructions for ridding the 

 Hydra of them. 



1 have often remarked that on the death of the 

 Hydra, and after it has been left to decompose iu 

 the water, a large number of minute animalcules 

 spring into existence, and surround the dissolving 

 parts of the body, evidently consuming it ; but they 

 are much smaller than those described as parasites, 

 and of a different form. In the " Micrographic 

 Dictionary," plate 24, is figured, No. 30, Stylo)iicMa 

 lanceolata, with side-view of the same, which appear 

 to me, when compared with those described as 

 parasites, to be the same ; if so, they are not ex- 

 clusively the parasites of the Hydra, but may be 

 found elsewhere. Still I have never observed any 

 of the other forms of Infusoria infesting the Hydra. 



Canterbury. James Fullagak. 



MICROSCOPY. 



Revolving " Dark Wells." — The following 

 modification of the ordinary " dark well " I find 

 very useful in the examination of certain opaque 

 objects with the Lieberkuhn, or Parabolic Illu- 

 minator. The following diagrams perhaps scarcely 

 need explanation. A is a front view of the "wells " as 



V y^v^ i 



Fig. 20. Revolving "Dark Wells." 



fitted to a short tube made to slide into the sub-stage 

 tube; B, the wells; C, section of ditto. The "wells" 

 differ somewhat from the ordinary form, being 

 slightly concave discs instead of short tubes"; the 

 concave surfaces of the discs I colour respectively 

 emerald-green, pale blue, and black. With dark 



