igo8.] R. E. Lloyd : Bathynomus giganteus. 85 



Besides these features there is, in the floor of the gizzard, a third pair of struc- 

 tures of greater importance ; for they are the only part of the organ which shows a 

 surface designed for the attrition of food. These are a pair of low ridges set trans- 

 versely in the floor of the gizzard, at the junction of the anterior and middle thirds 

 of its length. They lie beneath and in contact with, the anterior or muscular 

 ampullae, and are somewhat curved in accordance with the shape of those structures. 

 These transverse ridges are the only structures in the gizzard which can properly 

 receive the name of ossicles; they are covered with shining, yellow cuticle. The edge 

 of either ridge is slightly and irregularly lobulated (fig. 6). The cuticle of the lower 

 part of the muscular ampullae, where in contact with these transverse ridges, is some- 

 what thickened and is of a dark colour. Food material is evidently subjected to 

 a grinding process as it passes between these contiguous surfaces. 



The mid gut. 



The mid gut is plainly differentiated from the gizzard and from the hind gut, 

 both by its outward form and by the structure of its walls. It is 55 mm. in length, 

 and 15 mm. in breadth in the middle, where it is circular in cross section. It com- 

 mences in front within the head, where it projects in pouch-like form over the 

 hinder part of the gizzard : posteriorly it extends as far as the upper border of the 

 fifth thoracic segment, at which level it opens into the hind gut. The opening of the 

 mid gut into the hind gut is remarkable. In its general form the mid gut is 

 spindle-shaped, for its diameter gradually becomes less posteriorly. In the last 

 centimetre of its length it has the form of a slim cylindrical process, which is thrust 

 into the hind gut, in precisely the same way as the cervix of a mammalian uterus is 

 thrust into the vagina. The wall of the hind gut is so thin, that this process can be 

 seen dimly outlined within the cavity of the hind gut, before that organ is opened 

 (pi. X, fig. i). A median section through the length of both mid gut and hind gut 

 shows that the cavity of the mid gut is present though small in the anterior half of 

 this cervix-like process, but is merely potential in the posterior half. Furthermore, 

 the change from the open cavity to the potential one is sudden, and is accompanied 

 by a complete change in the character of the epithelium (fig. 2). 



The mid gut differs from both fore and hind gut in the much greater thickness of 

 its walls, which is more than a millimetre. At least four-fifths of the thickness of the 

 wall is composed of the internal epithelium, which is rugose and contorted to a 

 remarkable extent. Examination of this corrugated epithelium shows that it is 

 composed of ridges and furrows, which although very tortuous are in the main set 

 longitudinally. Moreover, the summits of these ridges are crossed by lesser trans- 

 verse grooves, dividing them irregularly into papillae. In some parts of the organ 

 the longitudinal arrangement is very plain, while in others the papillae are more in 

 evidence. The organ is not provided with a typhlosole. 



The hind gut. 



This is tne longest division of the alimentary canal, its length being in all 

 90 mm. It extends from the upper part of the fourth thoracic segment to the anus, 



