HABITS, ETC., OP CEREBRATULUS LACTEUS. 167 



either end of this cell, which extend across and fasten to 

 the pilidium wall. 



At first the fibres are simple, and contraction results in 

 pulling the pilidium walls closer together ; but in later 

 development they may be joined by other fibres, and the 

 motion then becomes compound, producing a general shrinking 

 of the body- walls. 



There are also one or more cells which sometimes accom- 

 pany the large retractor muscle, but whose fibres only extend 

 from the apical plate to the wall of the intestine (fig. 15). 



These are insignificant in size and strength, and probably 

 play an equally insignificant part in the motions of the larva. 

 Finally, there are muscles, consisting mostly of single cells, 

 which run from different points of the pilidium wall to that 

 of the intestine (figs. 13, 15). These seem to develop wherever 

 the cell is finally lodged, and their contraction produces move- 

 ment in both the walls to which they are attached. 



Being formed from a single cell, the body of the cell 

 usually remains visible for some time, but later it may travel 

 along to one wall or the other, and leave a simple strand 

 stretched across the intervening space. Here, then, we 

 witness the transformation of a cell, at first independent of all 

 the rest of the larva save the liquid in which it floats, into a 

 muscle-fibre fixed at both ends, and capable of contracting 

 under stimulus. 



3. CEsophageal Muscles. — There are two sets of these : 

 a. Gastro-oesophageal, consisting of delicate fibres 

 running diagonally between the adjacent walls of oesophagus 

 and stomach. 



Most of them consist of a single macromesencyte and its 

 branches ; they do not anastomose, and are so far apart that 

 they cannot fuse (fig. 14). They are not formed until the 

 seventh or eighth day, and are used chiefly in the dilation 

 (antero-posterior) of the ossophagus. They ai'e weak, and 

 scarcely worth a separate classification. 



h. Post- oesophageal, a thin muscular sheet, triangular 

 in shape, connecting the oral ectoderm with the posterior 



