STRUCTURAL PLAN OF HYDRA 255 



oval and cylindrical capsules are embedded. These capsules 

 are the nematocysts, organs of offence and defence possessed 

 by all members of the phylum Ccelenterata to which Hydra 

 belongs. The stiff hair-like processes projecting from the 

 surface are called cnidocils, and it is easy to determine that 

 each nematocyst has a single cnidocil associated with it. 

 If a drop of acetic acid or magenta is run under the coverslip 

 some of the nematocysts will be shot out and will assume the 

 forms shown in fig. 54, C, D, and E. There are three kinds of 

 nematocysts, the largest having the form of a pear-shaped sac, 

 the narrow end of which is produced into a short tapering 

 tube which narrows rapidly to form a long thread or filament. 

 The base of the filament is armed with three stiff projecting 

 spines and a number of smaller spines. The two other kinds 

 of nematocysts have shorter and thicker threads destitute of 

 spines, and the threads are usually curled up into a close cork- 

 screw-like spiral. Before the application of the stimulus the 

 thread in each nematocyst was introverted and coiled up inside 

 the cavity of the sac. To understand the meaning of " intro- 

 version " the reader should imagine an india-rubber sac pro- 

 longed at one end into a closed tube. If a thread were passed 

 up the inside of the tube and fastened to its inside at the tip, 

 by pulling on the thread the whole tube could be drawn back 

 into the cavity of the sac. If, then, the sac being closed and 

 filled with fluid, pressure was exerted on its walls, the intro- 

 verted tube would, in consequence of the pressure transmitted 

 to it, be everted and thrown out sharply to its original length. 

 There is no internal thread in a nematocyst but in other re- 

 spects its structure resembles that of the imaginary india-rubber 

 ball and tube. The contents of a nematocyst sac, however, 

 do not appear to contain a fluid but a semi-solid gelatinoid 

 material which stains deeply with haematoxylin and many other 

 dyes. This substance is hygroscopic, and it is due to the 

 absorption of water by it that the pressure inside the sac is 

 increased and the thread ejected. Hydra is able to sting and 

 paralyse its prey by means of its nematocysts, and it is probable 

 from the paralysing effects observed in the case of the water-flea 

 that these structures contain a poisonous secretion in their sacs. 

 It is clear, from what has preceded, that the structural plan, 

 the architecture of Hydra, is of the simplest kind. It is an 

 elongated sac closed at one end and open at the other. The 



