THIRD SUB-CLASS— CYCLOSTOMATA. 



Skeleton cartilaginous and notocJwrdal, luithout ribs and 

 ivithout 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, loithout 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 ^nd 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 vjith a suctorial mouth armed with teeth, 

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

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

 tinguished. Eyes present {in mature animcds). External 7iasal 

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

 duct terminates luithout perforating the palate. Seven branchial 



