CHORDATA. SUMMARY OF IMPORTANT FACTS 575 



Summary of Important Facts. 



1. The CHORDATA possess an axial skeleton, the notochord lying 

 between the ner\-ous system and the alimentary tract; a central nervous 

 system entirely on one side of the digestive canal, and gill slits extending 

 from the pharynx to the exterior. 



2. The Chordata are subdivided into Leptocardn, Tunicata, Entero- 

 pneusta, and Vertebrata. 



3. TheLEPTOCARDII are fishdikein form, have a notochord extend- 

 ing the length of the body, but lack heart, auditory organs, skull and verte- 

 bral column; the brain is rudimentary, the gill slits numerous. 



4. The TUNICATA have a notochord only in the caudal region. 

 The young is tadpole-like, but in most forms there is a metamorphosis in 

 which tail and notochord are lost. 



5. The body is usually enclosed in a tunic or mantle containing cellu- 

 lose; gill slits and an endostyle are present in the pharynx, the heart changes 

 in the direction of the flow of blood. The nervous S)'stem in its develop- 

 ment is tubular. In the Salpidse there is a typical alternation of genera- 

 tions between a solitary asexual and a sexual chain form. 



6. The ENTEROPNEUSTA are worm-like, with collar and proboscis ; 

 a diverticulum of the cUgestive tract is compared to the notochord; gill slits 

 occur in the pharynx; some undergo a metamorphosis in development, 

 the larva resembling those of Echinoderms. The pertinence of the 

 Enteropneusta to the Chordata is not certain. 



7. The VERTEBRATA are segmented animals ^^'ithout external 

 ringing of the body, but with metameric arrangement of internal parts 

 (myotomes, neurotomes, sclerotomes). 



8. A cuticular skeleton is absent, but there may be cornifications of the 

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



g. An axial skeleton is present, consisting of 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 

 appendages (anterior and posterior), which are usually present. 



1 1 . The central nervous system (brain and spinal cord) are tubular and 

 dorsal in position. The brain consists of five parts — cerebrum, 'twixt 

 brain, optic lobes, cerebellum, and medulla oljlongata. 



12. Eyes and ears are the most highly developed sense organs. 



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

 slits are present at least in the embryo, extending from the pharynx to the 

 exterior. In all terrestrial groups these are later replaced by lungs, 

 developed from the hinder end of the pharynx. 



