220 COMPARATIVE ANATOMY 



intestine, and partly surrounds the anterior end of the latter. 

 The pancreatic duct traverses the length of the gland, and 

 passes from the hinder ^nd of its ventral lobe to the intestine. 

 It runs back for a short distance in the ventral wall of the 

 intestine, and opens close to the commencement of the 

 spiral valve. The rectum is provided with a small tubular 

 rectal gland, which opens into its anterior dorsal wall. The 

 spleen, a ductless gland, having no opening into the alimentary 

 canal, may be mentioned here. It is a reddish organ, of 

 triangular shape, attached by a membrane to the bend of the 

 stomach. It sends forward a band-like prolongation along the 

 distal limb of the stomach, which is often mistaken by beginners 

 for the pancreas (fig. 51, .$/*/). It should be noticed that the mem- 

 branous folds suspending the alimentary tract from the roof of 

 the abdominal cavity, are very incomplete. The stomach is 

 suspended by a mesogaster ; there is no mesentery suspending 

 the intestine, but the rectum is suspended by a mesorectum. 

 Membranes attaching different regions of the gut to one 

 another, or other organs to the gut, are called omenta. In 

 the dogfish the following may be distinguished : a gastro- 

 hepatic omentum uniting the stomach and liver; a gastro- 

 intestinal omentum uniting the stomach and intestine ; and a 

 gastro-splenic omentum uniting the spleen and stomach. 



The spiracles and gill-slits are formed in the embryo as 

 outgrowths of the wall of the throat which meet and break 

 through the body-wall. The position of the internal openings 

 of the spiracles is shown in fig. 55. They lead into short 

 passages which pass in front of the hyomandibular cartilage 

 and open to the exterior. The anterior wall of the spiracular 

 passage is furnished with a rudimentary gill or pseudo-branch, 

 in the form of a few small vascular folds, whose presence 

 indicates that the spiracles are gill-slits of diminished size 

 which have lost their respiratory function. 



The internal openings of the gill-pouches are large vertical 

 slits, considerably larger than the external openings, and 

 curved in correspondence with the curvature of the lateral 

 walls of the pharnyx. Each slit leads into a capacious pouch, 

 narrow from side to side, but deep from above downwards. 

 The first four pouches have a gill or demi-branch on their 

 anterior and posterior walls; the fifth pouch has a demi-branch 

 on its anterior wall only. The shape and arrangement of the 



