12 A Guide to the Zoological Collections 



differing also from all vertebrates above the level of 

 Amphioxus. 



The specimens and dissections of Petromyzon (Lam- 

 prey) exhibited in Case 5 show that the body is worm- 

 like, without paired fins, but with unpaired dorsal and 

 caudal fins supported by fin-rays, and without scales. 



The mouth is a funnel, the walls of which are studded 

 with strong conical horny teeth, and, with the piston- 

 like tongue, forms an adhesive sucker. It is supported 

 by plates of cartilage, and is surrounded by fleshy lips. 



The gills are enclosed in pouches, which lie in a row 

 on either side of the gullet. The pouches — one of 

 which has been cut open to show the gill-leaves — open 

 on one side, indirectly, into the gullet, and on the other 

 side directly to the exterior : they are supported by a 

 cartilaginous basket-work, which has been dissected off 

 in the exhibited preparation. 



Immediately behind the gills is seen the heart, which 

 is of the ordinary fish type described further on. The 

 branchial artery is seen running from the heart between 

 the two rows of gill-pouches, and sends a branch to 

 each pouch. 



The skeleton is very imperfect. Instead of a back- 

 bone there is a simple elastic rod-like notochord : this 

 is enclosed in a sheath, from which a series of cartila- 

 ginous arches arise to cover in the spinal cord (dorsal 

 nerve-tube) that lies immediately above the notochord. 

 The relative positions of these organs are well seen in the 

 cross-slice of the body of a Lamprey in Case 5. The 

 skull is a mere capsule of cartilage. 



The eyes are small and are more or less embedded in 

 the skin. The nostril is a single pore placed in the 

 middle line of the head. 



The Lampreys are found in rivers and shallow seas 



