Compensatory Regulation. ig 



ment and finally by a discussion of the facts of compensatory 

 regulation as here exhibited. 



In a separate section (section four) a special series of experi- 

 ments on the regulation of the rate of differentiation of the oper- 

 cula in the Serpulid, Apomatus ampullifera, will be treated. 



I. Comparative Anatomy. 

 I. The Genus Hydroides. 



The opercula and branchiae of the genus Hydroides will ser^e 

 as the type in our description of the anatomy of these structures 

 throughout the family. The branchiae are brought in only 

 incidentally as our main purpose is to get the details of the struc- 

 ture of the opercula to serve as a basis for the regeneration and 

 regulation experiments to be described later. Unless other\vise 

 stated the description applies to H. dianthus. H. uncinata and 

 H. pectinata, living in the Mediterranean at Naples, were also 

 used in the experiments and the differences will be pointed out at 

 the close of the special description. H. dianthus is found on the 

 Atlantic coast of North America living attached to stones, mollusk 

 shells and other hard materials, from low water mark to a depth 

 of several fathoms. The worm lives in an irregularly twisted 

 calcareous tube which is attached by its side to the supporting 

 surface. The tube increases in size from the posterior end 

 anteriorly and is continually being built up at the anterior end by 

 additions from the special calcareous glands. 



The body of the animal is verv distinctly divided into the 

 thorax and the abdomen. The first-named region is marked by 

 the presence of the broad, flat fold of the thoracic membrane, 

 which is continued at both the anterior-ventral and the posterior- 

 ventral ends as a projecting membrane. At the anterior end this 

 membrane forms a collar which, except for a slight break on the 

 dorsal side, completely surrounds the head end. (See Fig. 5A.) 



Upon the anterior surface thus enclosed by the collar are located 

 the two semicircular rows of branchiae, one on each side of the 

 mouth, which apparently ser\-e on the one hand as organs of 

 respiration and on the other through their cilia as agents for the 

 creation of a current of water carrying food particles to the mouth. 

 The two rows of branchiae are placed on slight ridge-like eleva- 

 tions, the branchial ridges. These are not strictly semicircular 



