358 BULLETIN 120, UNITED STATES NATIONAL MUSEUM. 



ture, involving among higher forms degeneration in the alimentary 

 system, the sense organs and the locomotor organs, including the 

 muscies and the nervous mechanism connected with the locomotor 

 system and the sense organs. There is, therefore, a narrower range 

 of structural features within which the orthogenetic trends may ex- 

 press themselves. But Protozoa which live unattached within the 

 host, as do most of the endoparasitic Ciliates, lead about as active 

 lives as they would in a pond. They are protected from enemies 

 without becoming greatly degenerate. Many retain their original 

 alimentary apparatus and habit, being not parasites so much as 

 scavengers. The Opalinidae are hardly true parasites, for though 

 they live upon predigested food, to be sure, it is only upon sUch of 

 this food as is present in the fecal mass and was destined probably 

 to be thrown away. But the Opalinids have gone further than 

 Nyctotherus and Balantidium^ for example, in their adaptation to 

 parasitism, for they use predigested food and, in consequence, have 

 lost their ingestion apparatus and have doubtless modified their 

 physiological processes. But once these initial modifications were 

 secured, their further evolution and speciation has apparently been 

 due chiefly to orthogenetic trends free to an unusual degree from 

 hindrance or guidance from any selective conditions in their en- 

 vironment. The present condition, therefore, of the family is in 

 unusal measure an expression of the real nature of the organisms 

 themselves. Environmental opportunities have been limited and 

 environmental pressure has been less strong than in the case of 

 free-living organisms. 



THE ORIGIN AND THE SPREAD OF THE SEVERAL GROUPS OF THE ANURA. 



The problems and evidence as to the dates and places of origin and 

 the spreading of the Anura and their Opalinid parasites are so inter- 

 woven that it is difficult, in their discussion, to avoid passing con- 

 stantly back and forth between the hosts and their parasites. Data 

 from each side must be brought into the discussion of the problems 

 of the other. We have tried, however, in the first portion of the 

 discussion in this section to deal mainly with the parasites. In the 

 next few pages we will deal mainly with the hosts, and in the final 

 portion of this section we will treat some problems from the double 

 viewpoint. 



The PipmAE (fig. 222, p. 286). 



The Pipidae are usually considered the most archaic of the extant 

 families of the Anura. They are represented in northeastern South 

 America by one species, Pipa pipa^ the Surinam Toad. In Africa we 

 find two genera, Xenopus and Eymenochirus, which may be placed in 



