492 W. WOODLAND. 



say. The eoiitractility of the animal is then in all probability 

 tbe cause of the definite transverse disposition in the body- 

 wall of tbe long axes of the anchors. 



The shape of the anchor can also, I believe, be associated 

 to some extent with the contractility of the body-wall. The 

 body-wall of Synapta not only possesses bauds of longitudinal 

 muscle, but also a continuous sheet of circular muscle, the 

 presence of which latter implies that the body-wall can 

 diminish in diameter. Now the body-wall consists in the 

 main of two layers : the external thin dermal layer and the 

 internal, comparatively thick, sheet of circular muscle, and 

 between these two layers the elongated spicules lie, on a bed 

 of fibres, in a transverse position. If the circular muscle- 

 layer be imagined to contract, then it is evident that the 

 dermal layer, which at first is uniformly attached to the 

 muscle-layer, will be thrown into longitudinally-disposed 

 folds (see text-fig. 1), and that the position of these longi- 

 tudinal folds will, to some extent, be determined by the 

 spicules. In other words, the dermal layer will adhere to the 

 contracted muscle-layer in those parts of the circumference 

 which are devoid of spicules, but that, where spicules are 

 present, the dermal layer will, by the inevitable protrusion of 

 elongated spicules situated transversely on a diminished 

 circumference, be separated from the muscle-layer in the form 

 of pockets pushed out by the anchors. Consider the 

 mechanical aspect of the matter. Hods lying between two 

 connected layers forming the wall of a cylinder :md situated 

 transversely with regard to its long axis will, if their length 

 be adapted to the circumference of the cylinder, not project on 

 the surface at their extremities in any appreciable degree (text- 

 fig. 2). If, now, the inner layer only of the cylinder contract in 

 a considerable degree, it is evident that the extremities of the 

 rods will project on the exterior and that the outer layer will 

 be thrown into pocket- like folds enveloping the extremities 

 of the rods, but that in the portions of the circumference 

 situated between the rods it will still remain more or less 

 attached to the contracted inner layer, though in a longitu- 



