THE ELASMOBRANCHII. 



Ill 



ducts, but shed their contents into the abdomen, whence they 

 pass out by an abdominal pore. In the early stages of their 

 development the Lampreys present some singular resemblances 



Flo r<?,. — ^Vertical arivl longitixdinal section of the anterior part of the body of a Lamprey 

 yP^tramyzon maviiius): A, the cranium with its contained brain; a', section ol' the 

 ed^ of the cartilage marked a, in Fig. 30 ; Olf, entrance into the olfactorj' chamber, 

 wbii'.h is prolonged into the coecal pouch, o ; Ph, the pharynx ; Pr, the branchial chan- 

 nel, with the inner apertures of the branchial sacs ; Jf, the cavity of the mouth, with its 

 horcy teeth ; 2, the cartilage which supports the tongue ; 3, the oral ring. 



to the AmpJiihia. They also undergo a metamorphosis, the 

 young Petromyzon being so unlike the parent, that it was, un- 

 til lately, regarded as a distinct genus — Ammocoetes, But 

 the young Lampreys never possess external branchial filaments 

 or spiracula. 



The 3farsipohranchii are inhabitants of both fresh and salt 

 water. The Myxinoids are remarkable for their parasitic hab- 

 its — the Hag boring its way into the bodies of other fishes, 

 such as the Cod. No fossil Marsipobranchii are known. This 

 circumstance may, in part, be due to the perish ableness of 

 their bodies ; though horny teeth, like those of the Lampreys, 

 might have been preserved under favorable circumstances. 



III. The Elasmobranchii. — This order contains the Sharks, 

 the Rays, and the Ghimmra. 



The integument may be naked, and it never possesses scales 

 like those of ordinary fishes ; but, very commonly, it is devel- 

 oped into papillce, which become calcified, and give rise to 

 toothlike structures : these, when they are very small and close- 

 set, constitute what is called shagreen. When larger and more 

 scattered, they form dermal plates or tubercles ; and when, as 

 in many cases, they take the form of spines, these are called 

 dermal defences^ and, in a fossil state, ichthyodorulites. All 

 these constitute what has been called a '''' placoid exosJceleton ; " 



