484 BIOLOGY OF THE PROTOZOA 



while the cortex is often highly sculptured and fantastic as in the 

 Ophryoscolecidae. 



The aberrant Opalinidae are parasitic in Amphibia. Not only 

 are they astomatous, but in certain characters they differ widely 

 from other ciliates so that they have been variously placed in 

 classification. Hartog (1906), for example, placed them with the 

 Hypermastigida of the flagellates. Met calf (1918, 1923) includes 

 them as Prociliata sharply marked oft' from the remaining ciliates. 

 In view of the adaptive changes brought about by a parasitic mode 

 of life, it seems more probable that they are degenerate rather than 

 primitive types. There are invariably two or more nuclei but the 

 nuclei are identical with no indication of dimorphism. In the 

 nuclei, however, there are two kinds of chromatin according to 

 Leger and Duboscq (1904) and Metcalf (1909 and 1923). The 

 latter distinguishes these types as " macrochromatin " and "micro- 

 chromatin," the former in mitosis giving rise to band-form "macro- 

 chromosomes," the latter to "microchromosomes" in apparently 

 even numbers (from two to ten). The "macrochromatin" is re- 

 garded as functional in vegetative life and, like the macronucleus 

 of other ciliates, gives rise to chromidia (Neresheimer) or otherwise 

 fragments preparatory to absorption in the cell. The "micro- 

 chromatin" on the other hand is functional during sexual phases. 

 From these considerations it would appear that the dimorphic 

 nuclear conditions of ciliates generally is here represented by each 

 nucleus, but the hypothesis is questionable. 



In their sexual phenomena, also, the Opalinidae differ from the 

 majority of other ciliates. Individuals begin to divide rapidly with 

 decreasing size until minute forms result with one, two or more 

 nuclei according to species (Neresheimer, Metcalf). These encyst, 

 the cysts passing out with the feces. Tadpoles ingest the cysts 

 which open in the rectum, giving rise to the same type that had 

 previously encysted. These now multiply, ultimately forming mac- 

 rogametes and microgametes which fuse on contact. The zygote 

 has one nucleus at first which later gives rise to the binucleated or 

 multinucleated forms, although the exact manner has not been 

 described (Metcalf, 1923). 



Reproduction in ciliates generally is typically by binary cross- 

 division and involves a renewal of motile organs, at least this is the 

 case in forms with cirri, and MacDougall (1925) gives evidence to 

 indicate that cilia also are similarly renewed. It thus results that 

 motile organs of both products of cell division are proportionate to 

 the size of the young individuals. Old metaplastids, as pharyngeal 

 baskets, are discarded and new ones are formed in both halves. 

 Nuclear changes during division are quite varied, each species having 

 its own peculiarities of macronuclear condensation and reformation. 



Unequal division or budding, while uncommon among ciliates, 



