262 



THE FORMS OF CELLS 



[CH. 



the wrinkling which was invariably longitudinal in the body of 

 the cell is as invariably transverse in the narrow neck. The reason 

 for the difference is not far to seek. The conditions in the neck 

 are very different from those in the expanded portion of the cell : 

 the main difference being that the thickness of the wall is no longer 

 insignificant, but is of considerable magnitude as compared with 

 the diameter, or circumference, of the neck. We must accordingly 

 take it into account in considering the bending ynoments at any 

 point in this region of the shell-wall. And it is at once obvious 

 that, in any portion of the narrow neck, fl,exnre of a wall in a 



Fig. 87. Nodosaria scalaris, 

 Batsch. 



Fig. 88. Gonangia of Campanularians. 

 (a) C. gracilis; (b) C. grandis. 

 (After Allman.) 



transverse direction will be very difficult, while flexure in a 

 longitudinal direction will be comparatively easy; just as, in the 

 case of a long narrow strip of iron, we may easily bend it into 

 folds running transversely to its long axis, but not the other way. 

 The manner in which our little Lagena-shell tends to fold or wrinkle, 

 longitudinally in its wider part, and transversely or annularly in 

 its narrow neck, is thus completely and easily explained. 



An identical phenomenon is apt to occur in the little flask- 

 shaped gonangia, or reproductive capsules, of some of the hydroid 

 zoophytes. In the annexed drawings of these gonangia in two 

 species of Campanularia, we see that in one case the little vesicle 



