THE LOWER VBRTEBRATA. 199 



pair of grooves, curving round right and left of the mouth. 

 These grooves are lined by an epithelium that is partly ciliated, 

 partly glandular. In the endostyle there are four longitudinal 

 rows of gland-cells, separated by rows of ciliated cells. 



The gland-cells secrete a slimy fluid which the cilia drive 

 along the groove. To this slime most of the solid particles 

 brought in by the water-current will adhere, and while the 

 main part of the water passes out through the gill-slits into 

 the atrium, whence it is expelled by the atriopore, the solid 

 particles (mostly food) are carried on into the digestive 

 region. Thus the water-current is at once nutritive and 

 respiratory. In Amphioxus we find for the first time a reason 

 for that utilization of the anterior part of the alimentary 

 canal for respiration which is so marked a characteristic of 

 the Vertebrata. As we ascend to the dogfish, frog and 

 rabbit, we find the respiratory portion of the canal becoming 

 smaller and smaller, until in the rabbit the courses of the air 

 and of the food are almost completely separated. 



8. Intestine and Liver. Behind the last gill-slits the 

 canal becomes more nearly circular in section, and at first 

 there is a dilatation with glandular walls which secrete a 

 general digestive fluid: this is therefore often called the 

 " stomach." But into this opens a large caecum, the nature 

 of whose blood supply (9) shows it to answer to the liver. 

 The " stomach " into which it opens is, therefore, morpho- 

 logically the duodenum. The liver (hepatic caecum) is a 

 simple glandular sac, bearing much the same relation in 

 structure to that of higher vertebrates that the lung of the 

 frog does to the lung of the rabbit. It opens on the ventral 

 side of the i* stomach," but as it passes forward turns to the 

 right side, and appears in dissection lying alongside the 

 pharynx in the atrium. That the liver lies in the atrium 

 adds to the resemblance of that space to a coelom, but really 

 what is seen is not merely the liver, but the liver surrounded 

 by a narrow ccelomic space and a thin layer of connective 

 tissue and epidermis really the body-wall, which it has, 

 as it were, pushed out before it. 



No pancreas occurs in Amphioxus, though it may be 

 represented by the glandular walls of the " stomach." The 



