654 CEORDATA. 



heterocercal tail, lilacoid scales, gills covered, heart with arterial 

 cone, spiral valve in the intestine, no swim bladder. 



21. They are divided into Selachii (subdivided into Sqiiali, 

 sharks, and Raiaj, skates) and Ilolocejjhali. 



22. The Teleostei have bony skeleton, usually a homocercal 

 tail, usually cycloid or ctenoid scales, comb-like gills and operculum, 

 bulbus arteriosus, iisually jMoric appendages, and a swim blad- 

 der; no spiral valve. 



23. They are subdivided into Physostomi, Pharyngognatlii, 

 Acanthopteri, Anaoanthini, Lopholjrancliii, and Plectognathii. 



24. The Gajioidei form a connecting group ; they resemble the 

 elasmobranchs in the presence of a conus arteriosus and spiral 

 valve, and usually in the heterocercal tail; they are like the teleosts 

 in operculum and coml^-formed gills, swim bladder, and pyloric 

 appendages. They usually have fulcra and ganoid scales. 



25. The ganoids are subdivided into Chondrostei, with carti- 

 laginous skeletons, and Crossojoterygii and Holostei, with bony 

 skeletons. 



20. The Dipnoi have branchial respiration, occasionally the 

 swim bladder serves as lungs; heart with beginning division: nose 

 with choana. 



27. The Amphibia, in contrast to the fishes, have pentadactyle 

 appendages; in contrast to the reptiles, double occipital condyles. 

 They have bushy external gills, and lungs either persisting together 

 or succeeding each other, the young (larvas) breathing by gills, 

 the adult by lungs (metamorphosis!). The heart consists of two 

 auricles and one ventricle. 



28. The Amphibia are subdivided into G-ymnophiona. I'rodela, 

 and Anura; to these are added the extinct Stegoeephali (Laby- 

 rinthodonts). 



20. The Gymnophiona are blind and have lost the limbs. 



30. The Urodeles have many vertebra? and a well-developed tail. 

 They retain the gills permanentlj' (Perennibranchia), or at least a 

 gill slit (Derotrema), or they lose the branchial apparatus com- 

 pletely in development (Salamandrina) ; the metamorphosis is not 

 ]ironounced. 



31. The Anura have few vertebrae, no tail nor gills in the adult, 

 and a marked metamorphosis (the larva?, tadpoles, are furnished 

 at first with external, then with internal, gills, and with swimming 

 tail, but at first lack appendages and lungs). 



32. ('yelostomes, fishes, and Amphibia are grouped as Anamina 

 l)e<'ause of the lack of amnion and allautois; they are also called 



