CIIORDATA. SUMMARY OF IMPORTANT FACTS. 653 



7. The VEETEBRATA are segmented animals without ex- 

 ternal ringing of the bodj^, but with metameric arrangement of 

 internal parts (myotome, neurotome, sclerotome). 



8. A cuticuhir skeleton is absent, but there ma}' be cornifications 

 of the ejjithelium or ossifications in the derma (scales of fishes, etc. ). 



0. An axial skeleton is present, consisting of a notochord or 

 skull and vertebral column, which more or less completely replace 

 the notochord. 



10. There are two kinds of appendages supported by an axial 

 skeleton, the unpaired fins, occurring only in fishes and Amphibia, 

 and the paired ajipendages (anterior and posterior), which are 

 usually present. 



11. The central nervous system (brain and spinal cord) are dor- 

 sal in position. The brain consists of five parts — cereljrum, 'twixt 

 brain, optic lobes, cerebellum, and medulla oblongata. 



12. Of the sensory organs the eyes and ears are the most highly 

 developed. 



13. The respiratory organs arise from the entoderm (pharynx); 

 gill slits are jiresent at least in the embryo, extending from the 

 pharynx to the exterior. In all terrestrial groujjs these are later 

 rejilaced by lungs, developed from the hinder end of the j^harynx. 



14. The heart, consisting of auricle and ventricle, lies ventrally 

 in a j)ericardium. In gill-breathing species it contains only venous 

 blood, but with pulmonary respiration it is divided into venous 

 and arterial halves. The circulation is closed. 



1.5. The sexes are usually separate. In most species the ex- 

 cretory (nephridial) system forms the ducts for the reproductive 

 products (urogenital system). 



10. The reproduction is strictly sexual. 



17. In the Cyclostomata there is a primitive skull; but ver- 

 tebra3, paired fins, true scales, and teeth are lacking. The gills 

 are saccular and the nose is unpaired. There is no skeleton to 

 the mouth (no jaws). 



18. The true fishes (Pisces), like all other forms, have true 

 jaws (Gnathostomata). The fishes are further distinguished from 

 the Cyclostomes by the vertebral column (amphicade vertebra?), 

 l)y paired pectoral and ventral fins, scales, and paired nostrils. 

 They breathe by gills, and have a venous heart with auricle and 

 ventricle. 



10. The fishes are divided ijito Elasmobranchii, ftanoidei, 

 Teleostei, and Dipnoi. 



;'0. The EJas'inoliranchii have a cartilaginous skeleton, usually a 



