Ill] HYDRA 55 



have only to irrigate it with a little ten per cent, solution of common 

 salt, and from all parts of the skin we shall see first the threads 

 shoot out, and then the capsules follow. 



In the case of a fluid like salt solution, the stimulating action is 

 no doubt exerted over the whole surface of the animal, but an 

 examination of the tentacle when it is extended reveals an arrange- 

 ment for bringing about the explosion of the thread-capsule. The 

 surface of the tentacle is seen to be covered with little swellings, 

 in which are collections one might say, batteries of thread-cap- 

 sules (8, Fig. 17) ; and from the surface of the ectoderm, in which 

 they are embedded, delicate hair-like rods project out into the 

 water (3, Fig. 19). These rods are called cnidocils (Gr. wiSy, 

 a nettle ; Lat. cilium, an eyelash) and are the simplest form of 

 sense-hairs met with in the animal kingdom. If one of these be 

 touched, it transmits a stimulus to the cell containing the thread- 

 capsule, the cnidoblast (Gr. wiSy, a nettle; /3A.ao-T09, a sprout), as 

 it is termed ; in response to this stimulus the cell contracts, presses 

 on and explodes the capsule. 



In the first chapter it was pointed out that protoplasm, when it 

 effects movements, nearly always does so by contracting. Thus 

 the wrigglings of Euglena, the lashings of cilia and flagella, are 

 due to the contractions of myonemes, and even the emission of 

 pseudopodia may be due to a similar cause (p. 17). In the life 

 of Hydra the principal movements which occur are the shortening 

 and lengthening of the body and the tentacles (B, Fig. 16). 

 Now it has been found that in these movements, the shortening 

 is effected by the contraction of the ectoderm in a longitudinal 

 direction, and the lengthening by the contraction of the endoderm 

 in a transverse direction, in consequence of which the animal 

 is rendered thinner and longer. It has been further ascer- 

 tained, by the examination of very carefully prepared longitudinal 

 sections, that each ectoderm cell possesses at its base a tail 

 running vertically, which is embedded in the thin layer of jelly 

 sometimes called the structureless lamella or mesogloea 

 (Gr. /xeo-os, intermediate ; yXota, glue), which separates ectoderm 

 and endoderm (Fig. 20). The endoderm cells similarly possess 

 short tails, embedded in the jelly, but these run transversely. 

 These tails then are instances of the tendency of protoplasm, which 

 contracts regularly in one direction, to be drawn out into fibres in 

 that direction, or, in other words, we have before us the first step 

 in the conversion of an ordinary cell into a muscle cell. Cells 



