274 PRINCIPLES OF ANIMAL BIOLOGY 



Subclass DIPNOI. Fishes with a skeleton of cartilage and bone, and air bladder 

 functioning as a lung. Lungfishes. 



Class AMPHIBIA. Cold-blooded vertebrates breathing by means of gills in some 

 stage, skin usually not covered with scales, heart of three chambers. Salamanders, 

 toads, and frogs. (Figs. 84, 97, 98, 147, 153, 154, 159.) 



Class REPTILIA. Cold-blooded vertebrates usually covered with scales, breath- 

 ing throughout life by means of lungs. Lizards, snakes, crocodilians, turtles. (Figs. 

 152, 228-231.) 



Class AVES. Warm-blooded vertebrates with the body covered with feathers, 

 with the fore limbs usually modified as wings, and a heart of four chambers. Birds. 

 (Figs. 157, 245.) 



Class MAMMALIA. Warm-l)lc)oded animals which are covered with hair at some 

 stage and suckle the young, and have a diaphragm between thorax and abdomen. 

 (Figs. l.'')0, 190, 191, 196, 211, 232.) 



Subclass PROTOTHERIA. Egg-laying mammals. Monotremes. 



Subclass EUTHERIA. Viviparous mammals. True mammals. 



Invertebrate Groups of Uncertain Position 



Certain groups of invertebrates have not been assigned a definite 

 relation to other groups. Opinion ditl(>rs so widely as to their affinities 

 that they may well be kept out of the classification for the present. 



Mesozoa. Parasites apparently intermediate between the Protozoa and metazoa. 

 Not improbably degeneratt; relatives of the flatworms. 



Nemertinea. Terrestrial, fresh water and marine animals resembling flatworms 

 but with a proboscis, blood vascular system, and alimentary canal with two openings. 



Nematomorpha. Long thread-like animals with the body cavity lined with epithe- 

 lium, a pharyngeal nerve ring and a single ventral nerve cord. 



Acanthocephala. Parasitic worms with spiny proboscis, a complex reproductive 

 systcun and no alimentary canal. 



Chaetognatha. Marine invertebrates with a distinct coelom, alimentary canal, 

 nervous system and two eyes. 



Rotifera. Invertebrates with a head provided with cilia, usually a cylindrical or 

 conical body often with a shell-like covering, and a bifurcated tail or foot provided 

 with a cement gland. 



Bryozoa. Mostly colonial invertebrates resembling hydroids in form, with dis- 

 tinct ccelom, and with digestive tract bent in the form of a letter U. (Figs. 66, 129.) 



Phoronidea. A single genus of worm-like animals having tentacles and living in 

 membranous tul)es in the sand. 



Brachiopoda. Marine tentaculate animals with a calcareous shell, composed of 

 two unequal valves, a dorsal and a ventral. 



Gephyrea. Worm-like animals of doubtful affinities. 



