STIMULI AND THEIR ACTIONS 379 



2. The Actions of Mechanical Stimuli 



All changes in the pressure-relations of living substance 

 in its environment may be termed mechanical stimuli. The 

 effects of diminution of pressure have not thus far been 

 investigated in detail, hence only the effects of increase of pressure 

 are to be considered here. 



Increase of pressure can take place in various ways, ranging 

 from a light touch to a vigorous squeezing or complete crushing 

 of the living substance, from a brief shock to a continuous and 

 lasting pressure, or from an irregular shaking to rhythmically in- 

 termittent impacts, such as a tuning-fork produces. 



a. The Phenomena of Excitation 



Among the excitation- effects of mechanical stimuli upon the 

 phenomena of metabolism the clearest is that of the production of 

 substance, secretion, in unicellular organisms. Actinosphcerium, e.g., 

 when completely at rest, floats in the water, with many pseudo- 

 podia extended straight in all directions and evolving no secretion. 

 Absence of secretion is evident from the fact that ciliate Infusoria 

 belonging to the Hypotricha, which have cilia on their ventral side 

 only and by means of them run over objects in the water like lice, not 

 rarely walk along quietly upon the extended pseudopodia of the 

 Actinosphcerium without sticking to them. But if one of the 

 Hypotricha is actively swimming and bounds against a pseudo- 

 podium, the mechanical stimulus is sufficient to cause at the place 

 of contact the secretion of a viscous substance, which holds fast 

 the infusorian as prey. 1 A single strong shock likewise causes the 

 secretion of slime upon the pseudopodia, so that small particles 

 suspended in the water remain sticking to them. Such secretion 

 as the effect of mechanical stimulation is wide-spread in the naked 

 protoplasmic bodies of Ehizopoda. The slime becomes directly 

 visible in the large marine radiolarian Thalassicolla. It is possible 

 with little trouble to extirpate from the round body of Thalassi- 

 colla, which has the size of a pea, the central capsule, which is 

 pierced with extremely fine pores and contains protoplasm and 

 nucleus. If this be done without injury to it, after a short time the 

 capsule begins to regenerate into a complete radiolarian, i.e., to form 

 new pseudopodia, and gelatinous and vacuolar layers (Cf. Fig. 171). 

 After the pseudopodia have become extended like a circlet of rays 

 from the yellow spherical body, there is noticed between them an 

 extremely delicate, very liquid slime, which is excreted by the 

 pseudopodia and represents the rudiment of the new gelatinous 



1 Cf. Verworn ('89 X 1). 



