OPALmiD CILIATE INFUSORIANS— METCALF 579 



between the early Cretaceous and the middle Miocene, during the 

 mild climate m Antarctica, which we know from fossils prevailed 

 during the early Miocene and probably earlier. We regard Zelleriella 

 therefore, as a leptodactylid parasite of Antarctic origin in early or 

 middle Tertiary time. When the bar to northward spreading in 

 South America (see discussion of the Leptodactyhdae) disappeared, 

 leptodactyhds with their Zelleriellas passed northward to tropical 

 America, infected Hylas (sparsely) and Bufos, both already there, 

 with Zelleriella, and after the Isthmus of Panama was formed, in 

 the middle Phocene, they passed on across it in their original lep- 

 todactylid, and their new hyhd, hosts to Central America and the 

 Antilles which were at that time connected with Yucatan and probably 

 also with Honduras. Bufo also may have passed north across the 

 Isthmus at the same time. 



Zelleriella is subject to attack from parasites, which are found in 

 the cytoplasm, where they might be mistaken for stages in the nuclear 

 phenomena of the Zelleriellae (see fig. 44, c; and Metcalf, 1923a, 

 p. 135). I have observed these parasites from Brazil and Texas. 

 Several investigators (Stabler, 1933; Carini and Reichenow, 1935; 

 Chen and Stabler, 1935; Stabler and Chen, 1936; Chen and Stabler, 

 1936) have made detailed studies on these Endamoeha parasites. 



THE OPALININAE. COMPRISING THE GENERA CEPEDEA AND OPALINA 



We have already seen, while discussing Protoopalina, that there is 

 comparative anatomical and developmental evidence to show beyond 

 reasonable question that Cepedea developed from Protoopalina and 

 that there is indication that this occurred in Asia-Malaysia and 

 before communication of this region with Lemuria had been per- 

 manently shut off, for Cepedea is in Madagascar, the Seychelles, and 

 Ethiopia, as well as in India and Ceylon. Bufo is not in Madagascar 

 and the Seychelles today, so probably never was in Lemuria and 

 could not have been the original host of Cepedea. Ranids are today 

 the hosts of Cepedea in Madagascar and the Seychelles and probably 

 were its original hosts (Polypedatesf). They still carry in Asia- 

 Malaysia ProtoopaHnas of subgeneric group VIII from which Cepedea 

 evolved. We place the evolution of Cepedea, therefore, in Asia- 

 Malaysia-Lemuria, early in the Cretaceous period, in ranid hosts. 

 Metcalf (1923a) suggested origin in Lemuria, probably in gastrophryn- 

 ids. Cepedea apparently entered South America from Asia and the 

 north before its hosts for this migration had met Opalina, for Opalina 

 is not in South America; neither is Rana in South America, except 

 for one species, R. palmipes, which, probably since the middle PUocene, 

 has sparsely invaded the northernmost parts of the continent. Opa- 

 lina is now abundant in North America and Central America, chiefly 

 in Rana, Bufo, and hylids, but these have not passed south over the 

 Isthmus with Opalina. The most probable explanation seems to be: 



