STRUCTURE OF THE VERTEBRATES 9 



Phj'lum. Chord ATA 



Sub-phylum I. Hemichordata. Primitive animals with a poorly 

 developed notochord located in the anterior end. The type 

 order is composed of worm-like animals, the genus 

 Balanoglossus being the usual form for study. The other 

 two orders are aberrant and difficult for the beginner. 

 Sub-ph3dum II. Urochordata. A degenerate group in which 

 the embryos have a well developed notochord in the tail 

 region (iiro, tail). One order remains free-swimming and 

 retains the tail in the adult. The other two orders lose the 

 tail after the larval period. Embryology is necessary to 

 show the affinities of most of the group. 

 Sub-phylum III. Cephalochordata. A small group, closely 

 allied to the vertebrates, and often included with them as 

 the Euchordata ("complete" notochord). Amphioxus is 

 the type form, and is the simplest expression of verte- 

 brate characteristics known to naturalists. The notochord 

 extends from tip of head to end of tail. 

 Sub-phylum IV. Vertebrata or Craniata. Animals with verte- 

 brae surrounding, incompletely or completely, the noto- 

 chord. A cranium, or brain case, is also typical. The 

 sub-phylum is very large, and is divided into the follow- 

 ing classes. 



Class 1. Cyclostoviata are round-mouthed animals, as the 

 name indicates. The cyclostomes have no jaws, 

 no appendages, and very incomplete vertebrae. 

 The larva closely resembles Amphioxus. 

 Class 2. Pisces, or fish, have jaws, vertebrae completely 

 surrounding the notochord, and fins as ap- 

 pendages. All known fish differ widel}" from the 

 cyclostomes. 

 Class 3. Amphibia include the frogs and salamanders^ 

 animals with a ''double" life. The larva is a 

 water-living form with gills and many other fish- 

 like characteristics, and usually metamorphoses 

 into a lung-breathing, land-living animal. 

 Class 4. Reptilia. The reptiles are the turtles, lizards, 

 snakes and allis;ators. Thev are covered with 



