24 A Guide to the Zoological Collections 



In the small but highly interesting Sub-class of 

 Dipnoi (Case 23 & 24) the skeleton of the paired fins is 

 comparable to that of the limbs of higher Vertebrates ; 

 for it consists of a long segmented cartilaginous rod to 

 which the rays, when present, are articulated. The 

 dissection of the fore-limb of Ccratodus in Case 25 dis- 

 plays this. 



The Digestive System of Fishes is illustrated by two 

 dissections in Case 10, of a Sea-perch {Pristipoma) and 

 of a Ray {Trygon) in which the abdominal cavity has 

 been completely laid open and all the viscera labelled, 

 with the exception of the liver, which has been removed 

 for the sake of clearness. 



In the Sea-perch {Pristipoma) observe the large 

 mouth, the small tongue, the capacious pharynx with 

 four wide gill-slits on either side, the short gullet, the 

 Y-shaped stomach, the pyloric appendages, the single 

 loop of the small gut, and the straight large gut open- 

 ing at the vent. [The pyloric appendages are narrow 

 pouches that surround the intestine at its junction with 

 the stomach. Their microscopic structure does not 

 differ essentially from that of the neighbouring intes- 

 tine, into which they open]. 



In the Ray {Trygon) observe the U-shaped stomach, 

 the absence of pyloric appendages, and the very short 

 and wide intestine, which has been cut open to show how 

 its absorptive surface is increased by the broad spiral 

 pleating of its mucous membrane, known as the spiral 

 valve. Observe that the intestine opens into a pouch 

 common to it and the urinary and generative organs, 

 called the cloaca. 



