DISTOMUM CIRRIGERUM. 287 



At one end of tlie embryo (Fig. XI, 11) the cortical substance 

 is continued inwards into tlie body, and ultimately the layer 

 splits, and thus there is formed an embryo bent sharply on 

 itself, with its future dorsal surface outermost (fig. 12). This 

 band which splits to form the ventral surface should be 

 regarded as arising by a differentiation of the cells within 

 the embryo, and not as an ingrowth of the outer cortex. 

 The cortical layer now produces the relatively thin definitive 

 cuticle, which is very symmetrically studded with rows of 

 very tiny spinelets (Fig. XY). It is difficult at first to 

 imagine how these structures originate. They are far too 

 small for one spinelet to be produced by one subjacent cell ; 

 and further, the cells below the cortex never appear to be 

 arranged in rows or in any way symmetrically. It would 

 appear that the longitudinal and transverse rows of spinelets 

 are produced by the cortex protoplasm as a whole, and not 

 by the sum of the individual efforts of separate cells. 



Such an example warns us against regarding the body of 

 an organism as a community of individual units working 

 together for the common good ; but it points rather to the 

 conception of the body as a single machine of very great 

 complexity. 



The oral and ventral suckers next become distinguishable 

 as two nearly hemispherical bodies, composed of elongated 

 cells with large nuclei. 



The ventral sucker is formed at the bottom of the sheet of 

 cortex, which has not as yet split to form the future ventral 

 surface of the animal (Fig. XI, 11). 



Formation of the Gut. — At this period in the develop- 

 ment the gut-C£eca and pharynx become distinguishable. 

 The gut-casca are formed from flattened cells (Fig. XI, 11, 

 cfe.) which apparently become differentiated out of the 

 general cells of the embryo, as I have been quite unable to 

 demonstrate the previous existence of an endoderm. On the 

 right and left of the embryo flattened cells become so ar- 

 ranged as to form a very thin-walled and narrow sac on each 

 side (Fig. XYIII, ca\). The pharynx is seen at some little 



