Compensatory Regulation. 23 



"breaking joint" the cells of the connective tissue have more of an 

 embryonic character than elsewhere, having fewer and shorter 

 processes and less intercellular material. This distinction has 

 already been noted by Orley ('84) in his description of the con- 

 nective tissue of the opercular stalk in Serpula vermicularis. 

 He says, speaking of this tissue, "Die Modificationen dieses 

 Bindegewebes sind nach den Ortsverhaltnissen sehr verschieden. 

 hn mnersteji Theile des Stielcs wo dieser mit dem Kiemenlappen 

 zusammen hangt, findet man kleine weniger verzweigte Zellen 

 in der sehr sparlichen Intercellularsubstanz. Es ahnelt sehr der 

 embryonaler Form. Etwas hoher trifFt man bereits Zellen an, die 

 sich durch Grosze und durch die Zahl ihrer Auslaufe auszeichen 

 und eine gut entwickelte Intercellularsubstanz haben."^ The 

 significance of the differences of the regions w^ill be brought out in 

 connection with the experiments described later in the paper 



(P- 55)- 



The rudimentary operculum of H. dianthus has two well- 

 defined regions. The cells distal to the "breaking joint" are 

 distinctly embryonic in form and general character. Those 

 proximal to the breaking joint have among them well-developed 

 supporting cells of the type found in the branchiae and functional 

 operculum, though these cells are not as highly differentiated as 

 in the latter organs. 



A comparison of the two other members of the genus Hydroides 

 with H. dianthus brings out only slight differences in the charac- 

 ter of the opercula and branchiae. H. pectinata, however, has 

 pectinate secondary processes as opposed to the unbranched ones 

 of H. uncinata and H. dianthus. 



Any conclusion drawn from direct anatomical evidence must 

 emphasize a very close resemblance in the internal structure as 

 well as in the position of the opercula and branchiae. A similar 

 conclusion as regards the morphological worth of the rudimentary 

 operculum can be reached by a recognition of similarity in position 

 on the one hand and the nearly equal appearance of right and left- 

 handed individuals on the other. 



The functional operculum in Hydroides is therefore morpho- 

 logically a branchia which has formed an expansion at its distal 

 end and which has at the same time lost its respiratory pinnules. 



'Italics mine. 



