558 PROCEEDINGS OF THE NATIONAL, MUSEUM vol.87 



The drawings here published (fig. 125) are mostly self-explanatory 

 and little description is needed. Figure 125, a, is a typically Proto- 

 opalina-like larva; b shows the posterior nucleus entering mitosis; 

 c and d show stages with three and four nuclei, becoming Cepedea-like 

 larvae ; e and / depict individuals that have respectively five and six 

 nuclei and that are much flattened and represent the Opalinae latae 

 larvae, such as those in 0. larvarum that are much more emphasized. 

 In tadpoles of 0. virguloidea, the broad stage is very brief and occurs 

 in younger larvae with fewer nuclei. It seems quite possible that in 

 some narrow species of Opalma it may be found to be wholly sup- 

 pressed. Indeed it may perhaps be omitted from the life history of 

 some individuals of 0. virguloidea, for larvae in this stage are less 

 numerous in our slides than Cepedea forms with five to eight nuclei. 



Figure 125, h and i, resemble P. axonucleata. Larvae of this type, 

 with about 6 to 10 nuclei arranged along the midline of the body, are 

 very numerous and probably occur in the growth of every individual. 

 As they get older and the nuclei increase in number, the linear ar- 

 rangement of the nuclei is lost (fig. 125, j and k). By further multipli- 

 cation of nuclei, by growth and elongation, and by flattening, first at 

 the anterior end, then throughout the body, larvae resembling 0. 

 obtrigona arise (fig. 125, m). Comma-shaped forms with marked cur- 

 vature, especially of the anterior end, then arise, giving the adult form 

 (fig. 125, n), the further changes being increase in size and increase in 

 number of nuclei. 



The recapitulation of the phylogeny is remarkably complete, but 

 the broadening of the body occurs in this species earlier than in 0. 

 larvarum. The trend toward increase in number of the nuclei and 

 the trend toward flattening and broadening of the body are both pres- 

 ent in both 0. virguloidea and 0. larvarum, but the emphasis upon 

 broadening and flattening comes relatively a little earlier in the former 

 species. 



DIAGNOSTIC CHARACTERS IN OPALINIDAE 



There is much divergence in form of the body between different 

 genera, different species, and even different individuals of the same 

 species in the Opalinidae, but still there is a schematic plan upon 

 wliich they are built. Protoopalina is a spindle with a broad, rounded, 

 anterior end and a more tapering posterior part of the body. The 

 anterior end is bent and sometimes flattened; the posterior end may 

 have a long, slender point, or a much more abrupt point, or may be 

 rounded showing no point at all. There is much individual variation, 

 and this is more marked in preserved specimens, which may have been 

 killed in any of the diverse forms that the flexible bodies may assume 

 in life. For example, the bend in the anterior part of the body may 

 not appear. 



