THIED SUB-CLASS— OYCLOSTOMATA, 



Skeleton cartilaginous and notochordal, mthout rihs and 

 without real jaws. Skull not separate from the vertebral column. 

 No limbs. Gills in the' form of fixed sacs, without branchial 

 arches, six or seven in number on each side. One nasal aperture 

 only. Heart without bulbus arteriosus. Mouth anterior, sur- 

 rounded by a circular or sub-circular lip, suctorial. Alimentary 

 canal straight, simple, without coecal appendages, pancreas or 

 spleen. Generative outlet peritoneal. Vertical fins rayed. 



The Cyclostomes are most probably a very ancient type. 

 Unfortunately the organs of these creatures are too soft to be 

 preserved, with the exception of the horny denticles "with 

 which the mouth of some, of them is armed. -And, indeed, 

 dental plates, which are very similar to those of Myxine, are 

 not uncommon in certain strata of Devonian and Silurian 

 age (see p. 193). The fishes belonging to this sub-class may 

 be divided into two families — 



First Family — Petromyzontid^. 



Body eel-shaped, naked. Subject to a metamorphosis; in 

 the perfect stage utith a suctorial mouth armed with teeth, 

 simple or multicuspid, horny, sitting on a soft papilla. Maxil- 

 lary, mandibulary, lingual, and suctorial teeth vuiy be dis- 

 tinguished. Syes present (in mature animals). External nasal 

 aperture in the middle of the upper side of the head. The nasal 

 duct terminates without perforating the palate. Seven branchial 



