148 BULLETIN OF THE 



forming together the dorsal curve of the horseshoe. Brandt ('70, p. 22) 

 assigned a horseshoe shape to the crown of tentacles, but this has been 

 declared false by later investigators. 



This normal hippocrepian form is often distorted when the introvert 

 is only partially extruded, or when there is undue muscular contraction 

 within the soft mass of the fold itself, and it is always more or less 

 disguised by the secondary radial plaits into which the fold is thrown. 

 The relation of these parts will be easily understood by comparing 

 Figures 1, 2, and 3. It will thus be seen that the reflection of the ten- 

 tacular fold, with its deep radial plaits, brings into prominence regions 

 — the "triangular tentacles" of some writers — which alternate with 

 retreating portions, so as to impart to the margin the appearance of 

 being cut or toothed, especially if the contraction of the muscular ele- 

 ments in this soft fold has drawn it somewhat out of shape. In fact, 

 the description uniformly given by systematic writers has represented 

 the tentacles as a membrane with numerous marginal incisions. This 

 error is due in part to distorted specimens ; the true form may be said 

 to be crenate. 



Therefore one can speak of the formation of tentacles only in a gen- 

 eral sense. But the fold may be regarded perhaps as the simpler form, 

 from which, by the development of certain areas alternating with regions 

 of reduction, the more highly specialized digitate tentacles might be de- 

 veloped. Only the main folds are represented in Figure 2. These may 

 be much complicated by the appearance of subordinate plaits, until the 

 general plan is confused by a mass of detail. The more simple forms 

 proved, on microscopic examination, to have been the most successfully 

 killed, in that the muscular elements were in a more perfectly relaxed 

 condition. The aboral surface of the tentacular fold is concave, except 

 in the dorsal horns, where it is convex ; it has the same radial folds as 

 the oral surface with which it is approximately parallel. Numerous low 

 circular ridges traverse the aboral surface, and bear in varying number 

 the small papillae already mentioned. These ridges are not regular in 

 course or size, and evidently vary with the convexity of the tentacular 

 fold. In the midst of these, on the dorsal median line, can be found on 

 careful examination a small oval opening (Figs. 2, 3, can. o. ceb.). It is 

 often so hidden in the ridges of the aboral surface as to make its dis- 

 covery a matter of some difficulty. The opening measures about 1 by 

 0.5 mm., with its long axis transverse, and is surrounded by an evident 

 marginal ridge. This is the opening of the canal of the cerebral organ, 

 to be described later. 



