100 



ZOOLOC4Y 



SECT. 



II K 



A peculiar kind of spore-formation, specially adapted to the 

 requirements of an internal parasite, takes place in Opalina 

 (Fig. 76), a parasite in the intestine of the Frog. Binary fission 

 takes place (D, E, F), and is repeated again and again so rapidly 

 that the daughter-cells are unable to grow to the adult size before 

 the next division. The final results of the process are small bodies 

 (G), each with only three to six nuclei instead of the large 

 number characteristic of the adult. These become encysted 

 (H), and in this passive condition are passed out of the 

 Frog's intestine with its faeces, frequently being deposited on 

 water-weeds. All this takes place during the Frog's breeding 



season : the tad- 

 poles or Frog- 

 larvae feed upon 

 the water- 

 plantSj and in 

 doing so fre- 

 quently take in 

 the spores or 

 encysted Opa- 

 linae along with 

 their food. 

 When this oc- 

 curs the cyst is 

 dissolved by 

 the digestive 

 juices of the 

 host, and the 

 protoplasm of 

 the spore is set 

 free and be- 

 comes divided 

 up into club- 

 shaped bodies, 

 each with a 

 single nucleus 

 (I). These are 

 gametes, which 

 copulate in pairs, the zygotes growing into adult Opalinae (K). 



Conjugation, in the form of a temporary union accompanied by 

 interchange of micronuclei, has been described in Paramoecium 

 (p. 92), and takes place in many Ciliata. In others (e.g. Stylonychia 

 histrio) there is a complete union (copulation) of the two gametes. 

 In Vorticella union is also permanent, and takes place, not between 

 two ordinary forms, but between one of the ordinary stalked 

 individuals, or megagametes, and a free-swimming, small form, or 

 microgamete, produced a number together from one of the fixed 



FIG. 76. Opalina ranarum. A, living specimen ; B, stained 

 specimen showing nuclei ; C, stages in nuclear division ; D F, 

 stages in fission ; O. final product of fission ; II, encysted form ; /, 

 young form liberated from cyst ; K, the same after multiplication 

 of the nucleus has begun ; nu. nucleus. (From Parker's Biology, 

 after Saville Kent and Zeller.) 



