72 Charles Zeleny. 



though in different degrees, as compared with the ontogenetic 

 development. 



The discussion of the regulation of the process may be 

 referred to the general discussion of compensatory regulation in 

 the group of Serpulids, as given on p. 76. 



3. Probable Phylogenetic Development. 



The opercula and branchiae of the family Serpulidae furnish as 

 good a case of a morphological series as can be found within the 

 animal kingdom. There are all gradations between species with 

 no modification of the branchiae up to those with a degree of 

 opercular modification so great that no branchial characters can be 

 made out in the organ. Furthermore, in the ontogeny of the one 

 form studied (Hydroides), in which there is a high degree of modifi- 

 cation, each of the two opercula passes through a stage in which it is 

 to all appearances a functional branchia. The paleontological 

 evidence, however, is fragmentary. Our only knowledge is 

 obtained from the calcareous tubes and it is not always possible to 

 decide whether the animal inhabitant was or was not operculate. 

 Tubes evidently belonging to the genus Spirorbis are, however, 

 found as low down as the upper Silurian. 



The morphological and ontogenetic evidence leads us to the 

 probable conclusion that the ancestors of the present day opercu- 

 late Serpulids were non-operculate forms and that the opercula 

 arose in the course of phylogeny by the development of enlarge- 

 ments upon the branchiae which served to close the opening of the 

 tube in which the animal lived. 



Some speculations as to the origin of the asymmetry of the 

 opercula in the Serpulids may be permissible if it is recognized 

 that the course of the probable phylogeny can at present be no 

 more than guessed at. The existence of a morphological series 

 running from forms with no opercular modification of the branchiae 

 (Protula) through forms with a terminal enlargement at the end 

 of each branchia (Salmacina), others with two equal opercular 

 knobs one on each side of the median line attached to stalks 

 still retaining respiratory pinnules (Filograna), to still others 

 with a large operculum on one side and a small one on the other 

 (Hydroides, etc.) or with one operculum and that lateral in posi- 

 tion (Ditrupa, etc.) indicates that the early differentiations of the 



