OPALINID CILIATE INFUSORIANS — METCALF 611 



they have no organelles for the capture of prey. In such a secluded 

 habitat they have no need of protective devices, such as trichocysts. 

 No locomotor organs for rapid locomotion are needed in seeking prey 

 or in escaping enemies, and so their cilia remain feeble. There seems 

 to be no use for sense organelles and none seem to be found. With 

 sensation and locomotion reduced to a minimum, the neuromotor 

 organs are not emphasized. The excretory vacuoles in some Proto- 

 opalinas, on the other hand, give Httle indication of bemg reduced. 

 The simplification of structure in correlation \vith the sunplified life 

 in the secluded habitat and with an abundance of predigested food 

 furnished, itself allows less opportunity for the expression of divergent 

 character, that is for specific differences. Partial removal of the 

 animals from the action of natural selection allows such features as 

 do develop through the outworking and self-expression of their own 

 nature to persist. Natural selection does not suppress such slightly 

 divergent individuals as do arise, and thus the whole family, especially 

 the younger genera and subgenera, have but ill-defined species, e. g., 

 Zellerielta and the Opalinae angustae. In few, if any, other groups 

 of organisms is there better opportunity to study the almost unre- 

 stricted outworking of the tendencies inherent in the organisms 

 themselves. 



Study of the family in the light of these considerations shows us 

 that there are a number of such trends in them and their evolution 

 could be described in terms of the outworking and mterweaving of 

 these trends. (See Metcalf, 1927a.) Some of these trends seem 

 commonplace — a tendency toward flattening; a trend toward elonga- 

 tion ; a trend toward posterior pointedness and even the development 

 of a decided, pointed tail; a trend toward curvature of the body, 

 always in the same direction; a tendency toward developmg two types 

 of form in the same species, one slender, the other stocky, the differ 

 ence being in some cases so great as to have led to mistaking the two 

 types for separate species. The very remarkable trend is toward 

 delay in fission after the nucleus has divided, giving rise first to binu- 

 cleation and later, by the further suppression of additional divisions, 

 to multinucleation. The habit of delaying fission while characteristic 

 of the whole family is developed only to the point of producing binu- 

 cleation in the most archaic genus, Protoopalina, and in the Tertiary 

 genus Zelleriella, whfle in the Jurassic or early Cretaceous Cepedea 

 and in the Cretaceous Opalina the habit is emphasized to the pomt 

 of producing multinucleation. 



The Opalinidae, of course, are not the only organisms that show a 

 habit of suppressing fissions. Many plants fail to separate their 

 nuclei by ceU waUs, but this may not be a comparable phenomenon. 

 Among Protozoa certain genera or larger groups are regularly binu- 



