58 



MR JAMES RITCHIE : SUPPLEMENTARY REPORT ON 



An opposite pair of hydrothecse rests on each internode, six to seven or even ten 

 pairs being found on one colony. The hydrothecse are set slightly on front of the stem, 

 are always contingent in front for rather more than one-third of their total length, but 

 remain separate behind. The free portion diverges abruptly at a wide angle. Viewed 

 from the anterior aspect the sides of the adherent portion of a hydrotheca are parallel 

 to the long axis of the stem, while those of the free portion converge towards the 

 aperture. The contour lines, especially in the younger hydrothecse, are remarkably 

 straight and graceful. Beneath the innermost angle of the base there is a minute 

 chitinous thickening, and another projects from the base into the interior of the 

 hydrotheca. The latter appears to be roughly triangular in shape, the apex pointing 



d. 



n. 



FIG. 4. Sertularia heterodonta. (a) Anterior aspect of stem, x 60. (6) Lateral aspect of stem showing three-flapped 

 operculura. x 60. (c) Hydrothecaj with reduplicated margins, one regenerated margin bearing internal teeth, x 100. 

 (d) Gonotheca. x 50. 



into the cup, the side towards the centre of the stem being thickened and concave, 

 that remote from the centre sloping gradually till it merges with the hydrothecal floor. 

 This process represents a small ridge bounding the posterior edge of the opening 

 between hydrotheca and stem. The margin of the hydrotheca is divided into three 

 distinct and sharp teeth, the lateral pair the more prominent, the median and superior 

 individual tilted slightly upwards. There are three opercular flaps, difficult to 

 distinguish except when one is looking directly into the aperture of the hydrotheca. 

 Just within the margin and projecting from the inner wall are three prominent internal 

 teeth, triangular in shape, with blunt apices tilted towards the aperture. These 

 alternate with the marginal teeth. In hydrothecae towards the base of the stem the 

 superior pair appears occasionally to be undeveloped. Frequently the free portion of 

 the tube is much elongated by the reduplication of the margin, and in some cases a 

 new series of internal teeth is produced on the added portion (fig. 4, c). 



(ROY. 80C. EDIN. TRANS., VOL. XLVII., 80.) 



