Compejjsatory Regulation. 21 



Each branchia consists of a long axis bearing two rows of 

 secondary processes, the pinnules. (See Fig. 5D.) The axis is 

 continued for a short distance beyond the region of the secondary 

 processes as a slender tapering thread. The pinnule rows slope 

 inward so as to inclose a trough-like area, V-shaped in cross 

 section and with the cavity of the trough pointing inward, /. e., 

 toward the mouth. The surfaces bordering this area are ciliated 

 and it is along them that the food-bearing currents are formed. 



Near the dorsal end of one of the branchial ridges, not in the line 

 of the branchiae but dorsal to it, there is a stout, naked stalk of 

 approximately the same length as a branchia but bearing at its 

 distal end a funnel-shaped expansion. (See Fig. 5F.) The 

 whole organ constitutes the functional operculum. The edge of 

 the expanded portion is marked by teeth-like serrations, the 

 hollows between which are continued for some distance down the 

 outside. From the center of the terminal circular area within 

 this row of serrations there arises a group of secondary pro- 

 cesses, arranged so as to form a cup-shaped figure. The ends 

 of the processes are usually hooked and considerable foreign 

 material often clings to them. The whole organ serves as a very 

 efficient plug for the open end of the tube when the animal has 

 retired within for protection. An examination of the place of 

 attachment of the opercular stalk shows that it is located dorsal 

 to the first branchia or sometimes nearly opposite the interval 

 between the first and second branchiae. Near the base of the 

 stalk there is a transverse suture varying in distinctness in different 

 cases and which, as we shall see later, is a "breaking joint," an 

 important structure in the experiments. 



On the opposite side of the mid-dorsal line, and in a position 

 corresponding in all respects with that of the large operculum, is 

 a small organ consisting of a slender stalk with a slight terminal 

 enlargement. (Fig. 5E.) It also shows a distinct line of demarca- 

 tion between a darker colored more basal region and the lighter 

 remainder of its body. This small organ, the "pseudopercule" 

 of de St. Joseph is most commonly called the rudimentary oper- 

 culum. 



A study of the relative positions of the opercula is interesting. 

 An examination of 244 adult individuals of H. dianthus gave 139 

 or 57 per cent with the functional operculum on the right side and 

 105 or 43 per cent with it on the left. The distribution between 



