154 RADIATA. 



tions of water-plants, and may generally be obtained 

 by collecting some of these and placing them in a 

 glass jar of fresh water. When the water has stood 

 for some hours, the Polypes will be seen, on careful 

 examination, adhering to the sides of the glass. The 

 body of the animals is cylindrical, hollow, and fur- 

 nished with from six to ten tentacles, arranged in a 

 circle, in the centre of which is the mouth. The 

 tentacles are hollow, and communicate with the 

 cavity of the body. On examination with a high 

 power, the tentacles will be found to exhibit minute 

 oval sacs, containing a long fibre coiled up within 

 them ; and when the tentacles are touched by any 

 foreign body, the fibres are suddenly discharged. 

 These are the stinging or urticating organs. The 

 Hydra move very slowly ; but the body is very con- 

 tractile, and is often seen of various forms. When 

 a minute animal, as an Entomostracan, happens to 

 come into contact with the tentacles, these curve 

 around it, holding it firmly, and finally bringing it 

 to the mouth. It is then forced into the cavity of 

 the body of the animal, where it is digested, the re- 

 mains being discharged at the mouth. The move- 

 ments of the Hydra, when devouring its prey, form 

 a very curious and interesting spectacle. The Hydras 

 are propagated by budding or gemmation, also by 

 the formation of capsules in the walls of the body, 

 containing ova and spermatozoa. The young Polypes 

 formed by budding are represented in the figure, ad- 

 hering to the base of the parent. 



Sertuldria pumila (PL XI. fig. 15) is a marine 

 species, the polypidom being frequently found ad- 

 hering to Fuel and other sea-weeds ; it is about half 

 an inch long. The cells are opposite, pointed at the 

 ends, and with an oblique orifice. The tentacles are 

 fourteen in this species. In the summer large ovate 

 cells are found, arising from the polypidom; these 

 contain the eggs, and are called ovisacs or ovig'erous 

 vesicles. 



