CHAPTER XXV 



THE PROTOCHORDATA 



I HE Chordate Animals. — Referring to our metaphor of 

 the tree with its twigs as used in the chapter on 

 classification we find tlie fislies with the higher verte- 

 brates as parts of a great branch from whicli the lower twigs 

 have mostly perished. This great branch, phylum, or line of 

 descent is known in zoology as CJiordata, and the organisms 

 associated with it or composing it are chordate animals. 



The chordate animals are those which at some stage of life 

 possess a notochord or primitive dorsal cartilage which divides 

 the interior of the body into two cavities. The dorsal cavity 

 contains the great nerve centers or spinal cord ; the ventral 

 cavity contains the heart and alimentary canal. In all other 

 animals which possess a body cavity, there is no division by a 

 notochord, and the ganglia of the nervous system if existing 

 are placed on the ventral side or in a ring about the mouth. 



The Protochordates. — ilodern researches have shown that 

 besides the ordinary backboned animals certain other creatures 

 easily to be mistaken for moUusks or worms and being chordate 



. in structure must be regarded as offshoots from the vertebrate 

 branch. These are degenerate allies, as is shown by the fact 

 that their vertebrate traits are shown in their early or larval 

 development and scarcely at all in their adult condition. As 

 Dr. John Sterling Ivmgsley has weh said: "Many of the species 

 start m life Avith the promise of reaching a point high in the 

 scale, but after a while they turn around and, as one might say, 

 pursue a downward course, which results in an adult which 

 displays but few resemblances to the other vertebrates." In 

 the Tunicates or Ascidians (sea-squirts, sea -pears, and salpas), 



which constitute the class known as Tiuiicata or Urochordata, 



460 



