452 EVOLUTION OF ANIMALS Part V 



transverse division with the macronucleus and one or more micronuclei, de- 

 pending on the species, leading the division (Fig. 21.19). The old oral groove 

 goes with the anterior half and a new one is formed in the posterior half. A 

 new contractile vacuole forms in each part. Under favorable conditions the 

 process takes about two hours. At temperatures of 15° to 17° C. the animals 

 grow rapidly to mature size and at the end of about 24 hours each one divides 

 again. 



Conjugation is similar to fertilization, a mixture of nuclear materials from 

 two individuals thereby creating new hereditary combinations. Conjugation 

 rejuvenates the animals that take part in it, but it is not an essential process 

 and may not occur, Endomixis, the nuclear reorganization, that may take 

 place in single individuals, also rejuvenates the animals and stimulates division. 

 A description of conjugation is given in Chapter 18, Reproduction. 



Class Suctoria 



Adult suctorians have no cilia or other locomotor organs. Neither do they 

 have a mouth but take their food through tubular tentacles. The tips of these 

 tentacles are attached to other protozoans, thrust into their protoplasm which 

 then streams into the invader apparently by suction (Fig. 21.20). 



Suctorians are common in fresh water and salt; many live as commensals, 

 in fresh water attached to such various objects as algae, and the shells of 

 turtles; and in salt water, to sea weeds and hydroids. 



f. 2. 3. 



Fig. 21.19. Division of Paramecium caudatum: 1, micronucleus beginning di- 

 vision; 2, macronucleus lengthening, micronucleus in mitosis; 3, nuclear division 

 continuing; cellular division beginning; 4, two animals of the next generation. 

 Paramecia multiply only by division. Occasionally there is a temporary union 

 (conjugation) with exchange of nuclear material followed by the division of each 

 of the partners. An individual's life span is the period between divisions; in the 

 natural plan a lifetime is ended by a division not by death. 



