22 



AMERICAN HYDROIDS. 



There is no important change in the operculum or margin between the stage just described 

 and the completion of the hydranth. When tin- latter emerges into the outer world for the first 

 time it seems probable that it ruptures by purely mechanical pressure the line between the points 

 of the hydrotheeal teeth, or, to use our former simile, the line along the ridge polo of the tent, 

 the pushing tentacles being directed to that line by the .sloping inner surfaces of the opercular 

 flaps, and hence the cleavage along the line joining the summits of the teeth. After this rupture 

 has been effected further cleavage takes place along the line of junction between the slopes of 

 the hydrothecal teeth and the operculum, beginning at the top of the teeth (tig. 54). This 

 may include either or both sides of the "tent, "and will continue until there is room for the 

 egress of the hydranth, leaving the bottom of both Haps still attached to the hydrothecal margin. 



It is probable that Professor Levinsen would confirm the. above account, with the exception 

 of the last sentence. He holds that only the abcauline Hap is opened, the adcauline remaining as 

 the "collar" described above. This difference, however, is of great importance, as it is his 

 justification for regarding such an operculum as composed of a single Hap, while the present 

 writer maintains that it consists of two flaps. 



it will be conceded, I think, that the two flaps are identical in their origin and that they are 

 therefore strictly homologous anil similar structures. Levinsen claims that they differ in function, 



./.. ad 



r.r, 



KKl.ATION OF THE OPERCULUM TO THK EXPANDING AND EXPANDED HYDP.ANTH oh' SEKTUI.ARIA PUMILA. 



Fig. 54. Tuntai'h-s pushing aside the optTrular llapv <il>. ahraulitx 1 Ha].. 



Fig. 5f). Basi- nf rxpandrd hydra nth holding asid<- UK- oprn-illar Haps. ul>. ahi-alilim- flap; /'/, adraillim.' flap. 



Fiji, ."ii;. Vii'W of tlif opposit,- Mill., slimvillL' adi-ai'linr Map. in'. iidraulhlf Hap. 



the abcauline flap being a movable, lid and the adcauline a fixed ''collar. 1 " Mr. Paarmann's 

 investigation seems to prove that this is a mistake, and that "sometimes the adcauline piece, is 

 attached while the other is free, and sometimes the reverse is true. Often the sides of a flap are 

 attached for a greater or iess distance proximally while, they become free distally, the degree of 

 attachment varying greatly in the same species. In most cases both flaps are functional." 1 



In fig. 9, taken from a specimen preserved with the hydranth fully expanded, it can be plainly 

 seen that the abcaulino flap is not functional, and most careful scrutiny of the original under the 

 microscope does not reveal that it has opened at all. The hydranth is well expanded, however, 

 and so in this case the adcauline, flap must be the functional one, unless, indeed, it is so very 

 flexible and elastic that it allows of the passage of the hydranth without the lid being elevated at 

 all. In tig. 55 we have a case in which both flaps are plainly functional, and both are separated 

 from the hydrothecal margin well down toward the bottom of the tooth. It thus appears that 

 Mr. Paarmann was correct in his statement that both the abcauline and adcauline flaps are 

 functional, although neither is constantly so, and therefore the operculum of A', jn/milii ami numv 

 of its allies are properly called "two-Mapped."' The two flaps are the same in origin, are entirely 



'J. H. Paarmann, manuscript. 



- I'lic present writer has carefully verified the accuracy of Mr. I'aarniann's drawings l>y direct comparison with 

 the material from which they were taken. 



