26 Charles Zeleny. 



cula instead of eight less effective club-shaped enlargements. 

 The fact that when the animal is retracted the expanded portion 

 of one operculum occupies a position in front of the other may be 

 of importance in connection with a theory of the development of 

 asymmetry in these organs in other members of the group. 



d. Group IV. One Functional Operculum a?jd One Rudi- 

 mentary Operculum. Both on ends of Branchice. Pinnules present. 

 Examples — Apomatus, josephella. 



In Apomatus the next to the dorsal branchia on either the right 

 or the left side is expanded at its end into a globular almost trans- 

 parent operculum. The chitinous shell of the sphere itself con- 

 tains irregularly branched blood vessels, the green-colored blood 

 of which makes them very conspicuous. The branchia in a 

 corresponding position on the opposite side has a small ovoid 

 enlargement with a very pronounced network of blood vessels 

 containing distinctly pulsating green blood. Both these opercula 

 are placed at the ends of stalks which retain all the branchial 

 characters in an unchanged condition.- (Fig. 6d, e, f.) 



In a few cases the branchia occupying the place of the rudimen- 

 tary operculum ended in a tapering point instead of an ovoid 

 enlargement. There are usually about twenty pairs of branchia? 

 in the adult. Each of the two circlets breaks off very readily along 

 a definite breaking plane level with the anterior surface of the 

 head. The division plane is very pronounce'd and the break is 

 clean cut and takes place so readily that it is very hard to remove 

 the animal from its tube without causing it to throw off both of 

 the branchial circlets. 



e. Group V . One Functional Operculum and One Rudimen- 

 tary Operculum. Functional Operculum with Naked Stalk. 

 Rudimentary Operculum Not on End of Long Stalk. Examples — 

 Serpula, Crucigera, Hydroides. 



The description given above for Hydroides (p. 21) is sufficient 

 as a general characterization of this type. The functional oper- 

 culum may be either on the right or on the left side, the rudimen- 

 tary operculum in each case occupying the opposite position. The 

 opercula are not in the line of the branchiae but occupy a position 



^According to de St. Joseph ('94) the functional operculum appears on the left side and the rudimen- 

 tary, his "pseudopercule," on the right. He does not mention the possibility of the reverse arrangement. 

 The specimens which I examined at Naples showed a preponderance of .the right-handed condition. 

 (See p. 32.) 



