CLASSIFICATION 431 



test. This group possesses a notochord only in the caudal region. Ex- 

 ample, Tunicates. 



Subphylum HEMICHORDATA ( ). Worm- 



like chordates of doubtful systematic position. There is a projection 

 from the mid-dorsal region of the alimentary canal similar to a noto- 

 chord. These animals possess a collar and a proboscis. Example, 

 Balanoglossus. 



Subphylum CRANIATA or VERTEBRATA ( ). 



Chordates in which the notochord either persists or becomes invested 

 with cartilage. Vertebrates have a segmented spinal column. 



Class CYCLOSTOMATA ( ). Eel-like verte- 



brates without functional jaws or lateral appendages. Examples, hag- 

 fishes and lampreys. 



Class PISCES or GNATHOSTOMATA ( ). 



Fishes with a lower jaw and paired pectoral and pelvic fins, scales and 

 paired nostrils. The heart has an auricle, a ventricle, a conus arteriosus 

 and a sinus venosus. 



Class AMPHIBIA ( ). Cold-blood vertebrates 



breathing by means of gills at some stage of their life-cycle. Skin not 

 usually covered with scales. Three chambers in heart beside the conus 

 arteriosus and sinus venosus. Frogs, toads, newts, and salamanders. 



Class REPTILIA ( ). Cold-blooded verte- 



brates breathing by means of lungs throughout their life-cycle. Usually 

 covered with scales. Lizards, snakes, crocodilians, and turtles. 



Class AVES ( ). Warm-blooded vertebrates, 



whose body is usually covered with feathers and the fore-limbs modified 

 far wings. Heart of four chambers. Birds. 



Class MAMMALIA ( ). Warm-blooded ani- 



mals with hair covering at some stage in their life-cycle. They suckle 

 their young and have a diaphragm between thorax and abdomen. 



Subclass PROTOTHERIA ( ). Egg-laying 



mammals. Example, monotremes, such as the Australian duck-bill. 



Subclass EUTHERIA ( ). Mammals which 



give birth to living young. These are the true mammals. 



Division DIDELPHIA or METATHERIA ( ). 



These are the marsupials, such as the opossum and kangaroo. 



Division MONODELPHIA ( ). These are the 



placental animals which are nourished in the body of the mother through 

 a true placenta. 



INVERTEBRATE GROUPS OF UNCERTAIN POSITION 



Certain groups of invertebrates have not been assigned a definite 

 relation to other groups. Opinion differs 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 degenerate relatives of the flatworms. 



