iS8 



THE FOOD OF ANIMALS 



are microscopic structures imbedded in the body -wall, and 

 particularly abundant on the tentacles, where they are often 

 aggregated into " batteries ", giving rise to a roughened ap- 

 pearance as seen under strong magnification. Each of them 

 (fig. 412) is a highly-modified cell, and before use 

 looks like a little vesicle or bladder within which a 

 thread is coiled up. The bladder contains a poi- 

 sonous fluid, and its elastic wall is covered by a thin 

 layer of living matter or protoplasm, from which a 

 stiff" " trigger-hair " projects to the exterior. The 

 layer of living substance covering the vesicle has 

 been described as in connection with a nerve-fibre, 

 and this again with a nerve-cell. The coiled thread 

 is hollow, and can be shot out to the exterior by 

 pressure of the fluid in the vesicle, the motive power 

 being supplied by the living layer, which is capable 

 of contracting so as to exert a squeezing action. 

 During the process the thread is necessarily turned 

 inside out (see fig. 412), and the first part to emerge 

 is provided externally with a varying number of back- 

 wardly - directed spines. The apparatus is under 

 nervous control, and the animal undoubtedly exerts 

 discriminative power, for the contact of, say, a sand- 

 grain with the trigger-hair leads to no result. But 

 if suitable prey, such as a shrimp, is the disturbing 

 influence, the stinging-cell "explodes", i.e. the thread 

 is rapidly protruded to the exterior, its spiny base 

 inflicting a wound, and holding fast like a barbed 

 arrow. Part or all of the rest of the thread is then 

 ejected into the wound, and the poisonous fluid is 

 able to exert its full effect. Should the victim struggle, 

 and it often does, other tentacles are brought up to the 

 attack, and it is eventually overcome by the action of 

 a sufficient number of stinging-cells. These differ 

 from the adhesive knobs of a Comb- Jelly in the fact 

 that they can be only used once, and after explosion are cast off, 

 being constantly replaced by the growth of others. 



Corals. — Anemones are devoid of any hard parts, but their 

 relatives the Corals (fig. 413) are well-endowed in this respect. 

 The simplest Corals are solitary, and each of them has its base 



Fig.412.— Thread- 

 cells (greatly en- 

 larged}. The one 

 on the right is ex- 

 ploded 



