1903.1 NATURAL SCIENCES OF PHILADELPHIA. 363 



ON THE MORPHOLOGY OF THE ROTATORIAN FAMILY FLOSCULARIID^. 



BY THOMAS H. MONTGOMERY, JR.^ 



The thoughts which group themselves about the theories of larval 

 homologies, and in particular that one which regards the trochophore 

 stage as the recurrence or at least parallelism of an ancestral phyletic 

 form, comprise one of the many inducements to investigate the anat- 

 omy of the Rotatoria. For it is generally maintained by those workers 

 who uphold the trochophore theory in its logical sequences, that the 

 adult Rotatorian and the trochophore larva show close similarities. 

 One line of the approach, then, to test the correctness of the ideas of 

 such larval homologies, is to extend our knowledge of the structure of 

 the Rotatoria. A huge literature has grown up around this group of 

 animals, but with the exception of a very few detailed monographs 

 the writers have contented themselves w^th the description of the exter- 

 nal form. This is the more remarkable since the Rotatoria lend them- 

 selves better than most forms, thanks to their great transparency, to 

 the study of fine details of structure in the Hving animal. 



Those who have not occupied themselves especially with the anatomy 

 of this group make the common assumption that all Rotatoria show 

 essentially the same type of structure. But this is erroneous for two 

 reasons : first, because in Nature there do not occur types of structure, 

 but gradations of structure; and second, because the Rotatoria evince 

 very great differences among themselves. For the bearing of the group 

 as a whole upon the views of larval homologies it is first necessary to 

 examine the various Rotatorian groups comparatively, in order to 

 determine which of them is the most generahzed or primitive; then 

 this group should be compared with the various kinds of trochophore 

 larvae. This is a problem to be attacked from more sides than the 

 purely morphological, for at once appears the striking fact that the 

 Rotatoria are for the most part fresh-water forms, while the trocho- 

 phore larvae are characteristic exclusively of marine groups. Of the 

 living Rotatoria, are the most primitive forms found in the sea or in 

 fresh water? And of them, are the pelagic forms more primitive than 



1 From the Zoological Laboratory of the University of Pennsylvania, Phila- 

 delphia. 



