THE ANNULUS OF A MEXICAN CRAYFISH. 



129 



outside as a translucent object is represented in Fig. 7. The 

 line of the mouth coming around from the left face makes an 

 imperfect S which is much shorter than the inner edge of the 

 pocket walls, indicated as a continuation of the outside shell. 

 Through the thick exoskeleton runs only a narrow crevice the 

 outer mouth of which is shown as the black iine or suture and 

 the bottom of which is indicated by two parallel lines which cross 

 the suture at two points. The sides of the chink, or cavity of 

 the pocket, are diagrammatically represented by ruled lines. 

 The cavity of the pocket thus runs in from the mouth at first 

 obliquely downward and to the observer's left then slightly up- 

 ward and then downward again and toward the observer's right. 

 In other cases (Fig. 10) the middle loop toward the apex of 

 the annulus is much more pronounced. The simple nature of 



Fig. 8. 



Fig. 



the pocket is shown in the lengthwise section of the annulus (Fig. 

 8) which shows the narrow cavity of the pocket of a dextral an- 

 nulus near its origin toward the right face of the annulus. 



Other sections show the same simple pocket, whenever cut at 

 right angles to its twisting course. In Fig. 9 is shown, enlarged, 

 the terminal part of a dextral pocket cut near its median termi- 

 nation where it slopes away from the surface toward the median 

 plain and also down toward the base of the annulus. Here as 

 elsewhere the pocket is seen to be really an invagination of the 



