28 THE PROTOZOA THE DAWN OF LIFE 



the same size and structure ; here the outstanding feature is the 

 interchange of the micronuclei, which exerts a revivifying or fecun- 

 dating effect upon the conjugating individuals. In the case of a 

 sedentary 'Infusorian called Spirochona, which is to be found 

 attached to the gills of Gammarus (one of the Amphipod crus- 

 tacea), no external features of differentiation have been observed, 

 and possibly do not exist between individuals capable of con- 

 jugating with each other. Two apparently identical individuals 

 will bend towards each other and join by their oral surfaces, one of 

 the individuals ultimately separating from its basal support and 

 becoming partially or wholly absorbed by the other one, thus 

 ceasing to exist as an individual. 



In the graceful Vorticella or Bell-animalcule and some of its 

 allies, the difference between individuals that can conjugate is 

 well marked prior to the process taking place. The Bell-animal- 

 cule is a very common inhabitant of ponds, attaching itself to 

 the leaves and stems of the submerged vegetation by a slender 

 spirally contractile stem. Its body is slightly oblong, shaped 

 like a bell or wine-glass, the slender stem of which forms the con- 

 tractile stalk. The rim of the bell encloses an elevated disc of 

 protoplasm, between which and the rim on one side is the open- 

 ing of the mouth leading to the gullet. Cilia run in a spiral band 

 round the rim of the bell, round the edge of the disc, and down 

 into the gullet, producing active currents in the water by their 

 motion, and so capturing and carrying down into the gullet par- 

 ticles of organic matter upon which the Infusorian feeds. Mul- 

 tiplication often takes place by division of the bell-body longi- 

 tudinally, one of the halves being set free furnished with an addi- 

 tional posterior circlet of cilia by means of which it swims actively 

 about for a time, eventually becoming attached to some base by 

 a spirally contracting stalk. After division, the remaining un- 

 detached individual may rapidly divide into eight small units 

 male cells which, becoming detached, swim away by means of 

 their cilia, but do not ultimately settle down as the normal seden- 

 tary stalked form. The sedentary stalked form, which may be 

 called the female, is not capable of conjugating with another 

 individual of the same kind, but only with one of the small free- 

 swimming forms (males) that are periodically produced. Nor 

 can these free-swimming forms conjugate with each other, but 



