ro.i323. STRUCTURAL FEATURES OF IIOMOTRYPA—BASSLER. 569 



aeing set aside for that purpose. In Ceriocava ramosa of the Creta- 

 3eous (see Plate XX, figs. 5, 0), ovarian-like A-esicles, very similar to 

 3ystiphrao-nis, have been found, and this fact leads to the belief that 

 the cystiphragm also had a reproductive function. Hence the restric- 

 tion of these structures in this species to the zoa>,cial tubes of the 

 maculte onl}^ may have some significance. 



THE ACANTHOPORE. 



The majority of the species of IIomotTypa exhibit these spine-like 

 structures — the acanthopores. The ordinary acanthopore when show- 

 ing at the surface is seen to be a blunt spine situated on the cell wall, 

 usually at the angle of junction of adjoining zooecia. 



Tangential sections show that this spine is composed of concentric 

 rings of laminated tissue inclosing a minute, round canal. A vertical 

 section brings out the fact that the spine is not a mere surface orna- 

 ment, but that it is a tube inclosed in the wall substance of the zooecium; 

 that this tube is developed generally with the mature region and con- 

 tinues as an independent structure to the surface. In some species 

 they are well developed and here exhibit their structure most clearly. 

 The acanthopores of // nodulosa and IT. cyUndrica of this paper show 

 all that has been learned concerning these structures. That of JI. 

 nodulosa viewed in a tangential section (Plate XXllI, fig. 7) difl'ers from 

 the ordinary form in having a comparatively large central space, the 

 diameter of this sometimes being as much as one-half that of the entire 

 structure. A vertical section (Plate XXIII, figs. 5,6) shows clearly that 

 the acanthopore is not only a tube, but that this tube is also crossed by 

 (thin transverse partitions about the tube diameter distant from each 

 ,other. Such a section also shows that instead of being limited to the 

 mature region, the acanthopore may develop in any part of the 

 immature region, pass through in turn both this and the mature region, 

 and then, instead of stopping with the zoarial growth to which it belongs, 

 .continue through the inmiature and mature regions of an incrusting, 

 secondary growth of the species if this be present. This section also 

 shows that the concentric rings seen in tangential sections surrounding 

 the central space are the cut edges of overlapping, conical layers of 

 , tissue forming the wall of the tube. The acanthopore of H. cyUndrica 

 (Plate XXII, fig. 10) is interesting because it shows to how great an 

 extent these conical layers may be developed and, in contrast with 

 , preceding species, how minute the central cavity may be. Yet even 

 , in this species, in which the diameter of the tube is generally less than 

 ; one-twentieth that of the entire acanthopore, the transverse partitions 

 may be seen under favorable circumstances. 

 ; Possessing such structure, it must be conceded that the acanthopores 

 were of no little importance to the zoarium. Just what their function 



