12 AMERICAN HYDROIDS. 



Hartlaub 1 says that lie lias seen the blind sack filled with food particles, and suggests that it 

 may function as the stomach of the hydranth. It is not improbable that it may share this 

 function with the rest of the body cavity of which it is merely a divarication, but I see no proof 

 that it assumes the full burden of the digestive function. This writer also calls attention to the 

 fact that the adcauline wall of the sack has an endodermal lining of columnar epithelium like 

 that of the body cavity in general, while the abcauline wall is lined with ordinary endodermal 

 cells. (Fig. 15.) He suggests that the blind sack may have something to do with the renewal of 

 the hydranth, but does not support this suggestion with any definite proof. He doubtless infers 

 that the undifferentiated endoderm of the sack must be capable of some function other than that 

 performed by the columnar epithelium, and this may have given rise to the suggestion regarding 

 the connection between the blind sack and regeneration. He says that this structure is absent 

 from the Campanularida> in general, and believes that it constitutes a good systematic character 

 bv which that group may be separated from the Sertularidse. It is doubtless true that this 

 important structure has been altogether too much neglected by systematists. The present writer, 

 however, desires to make a critical study of it in connection with the other groups before adopt- 

 ing it as a criterion for the division of family groups. 



In several species of Sertularella, as .V. dichntoiiKt, and in the Desmoscyphus group of 

 S, liiiliirin* there is an internal ridge on the abcauline wall of the hydrotheca that corresponds 

 to the intrathccal ridge in the Plumularidse and appears to be for the firmer anchorage of the 

 protractor. (Fig. UH, /.) 



But one other feature pertaining to the hydranth need be discussed here, and that is the 

 structures that are supposed to be muscles for the closing of the operculum. I have been unable 

 to find them as a constant feature in any one species. Hartlaub figure* them as present in 

 Sertula/rdla r/"/// ', but they are certainly not at all constant in that species. 



While I have found a number of cases which 1 at first was inclined to consider as opercular 

 muscles, they were found on closer examination to end on the margin of the hydrotheca, and not 

 to reach the operculum at all. In such case's it is reasonable to interpret the structures rather as 

 protractors than as opercular muscles. (See figs. 17-10, <> ///.) In other cases the muscular bands 

 end freely in the upper part of the hydrotheca] cavity, as if they had been torn from their attach- 

 ments. These may be opercular muscles, but until they are found connected direct!} 7 and 

 definitely with the operculum the writer believes that it is wisest to refrain from ascribing to 

 them a definite function in connection with the opercula. It must be remembered that the 

 hydranths at times send forth all sorts of projections from the ectoderm toward the hydrotheca] 

 walls, and doubtless these are occasionally attached to the operculum. But we have as yet no 

 evidence that such attachments are permanent or constant, as are the protractors described above. 

 Hartlaub, who copied the figure mentioned above from Allman. is not at all convinced that there 

 are such things as retractors of the opercula. and suggests that Allman was mistaken in his 

 interpretation, as it often happens that one or more tentacles of a retracted hydranth remain with 

 their tips attached to the opercula. The present writer has not seen instances of this. In one 

 case (fig. 18) there is a structure that looks a good deal like a retractor of the operculum. The 

 figure, was taken from a section, and it appears that the long sarcodal process from the hydranth 

 is directly attached to the operculum, but it may not be a retractor at all, but simply one of the 

 many processes thrown out by the hydranth under certain conditions, particularly when the latter 

 is about to begin the process of disintegration. The mechanical necessity for retractors to the 

 operculum does not seem at all evident. The valves are so arranged that they would naturally 

 fall back into place upon the retraction of the hydranth, and this action is probably aided and 

 hastened by the elasticity of the chitinous material of which they are composed. 



In size the Sertularian hydranth does not differ appreciably from that of the I'lumularida 1 , 

 although they average somewhat larger. Although they are almost always retracted in preserved 

 specimens they are still available for studj*, while those of the plumularians are usually entirely 

 absent in specimens preserved in alcohol or formalin. Hartlaub says that the proboscis is 



1 Revision der Sertularella-Arten, 1900, p. 11. 



