A STUDY OF METAMERISM. 427 



riorly. In other words, there is a continuous body 

 cavity (coelora) lying between the coils of the septum, 

 and this cavity is continuous from the anterior to the 

 posterior end of the spiral. 



Fig. 66 shows a similar spiral, the only difference between 

 this and the last being that in the former (fig. 65) the anterior 

 end of the spiral abuts against the septum in front, while here 

 it ends freely. 



Fig. 67, A,B,c, shows a similar but longer spiral. Opened 

 dorsally the septa are found as in b, and it will be seen that 

 they follow the course of the outer spiral. In the middle line 

 the septa over the intestine show a tendency to irregularity 

 (even more so than the figure shows). In c the septa are 

 drawn as seen from below. They run obliquely over the lower 

 wall of the digestive tract. In both b and c the figures were 

 drawn from preparations made by dissecting free the digestive 

 tract and its attached septa from the body-walls. 



These examples will suffice for the regular forms, where 

 surface spirals and septal spirals agree ; but this, as stated 

 above, is not always the case. 



In fig. 68, A B, we find on the surface a spiral shown in a ; 

 that belongs to category x (PI. 40, fig. x). Opening the worm 

 we find that the septa are also spirally arranged, but the spiral 

 is shorter than the surface spiral by one turn, although both 

 begin and end on the upper surface. 



Fig. 69, A B, shows a parallel case, where the internal spiral 

 is shorter than the external. In this case the septal spiral is 

 shown only in its somatic attachment. 



In fig. 70, A,B,c, we find a surface spiral like the last. We 

 find the septa arranged dorsally as shown in b, and on the 

 ventral as shown in c. It will be seen that the external 

 and internal (septal) spirals belong to different forms (cate- 

 gories). The external belongs to category xi (PL 40, fig. xi), 

 while the septal belongs to category ix (extended), PI. 40, 



fig. IX. 



Fig. 71 shows a form in which the external line between two 

 metameres failed to meet in the mid-dorsal line. In dissection 



