400 COLLEGE ZOOLOGY 



atrial cavity and reach the exterior through the atriopore. Fer- 

 tilization takes place in the water. The early development of 

 the egg of Amphioxus was described in Chapter III (pp. 87 to 89), 

 and is illustrated in Figure 5 1 . For a detailed description of the 

 embryology of Amphioxus, the student is referred to Willey's 

 Amphioxus and the Ancestry of the Vertebrates and to advanced 

 text-books of zoology. 



4. SUBPHYLUM IV. VERTEBRATA: INTRODUCTION 



The VERTEBRATA are animals with an axial notochord at some 

 period in their existence. This notochord persists in some of 

 the lower vertebrates, but is modified by an investment of carti- 

 lage which becomes segmented and constitutes the vertebral col- 

 umn. In the higher vertebrates the vertebral column is made up 

 of a series of bodies called vertebrae, and the notochord disappears 

 before the adult stage is reached. The vertebrates are the lam- 

 preys, hags, sharks, rays, chimaeras, fishes, frogs, toads, sala- 

 manders, lizards, snakes, crocodiles, turtles, birds, hairy quadru- 

 peds, w r hales, seals, bats, monkeys, and man. Seven classes of 

 vertebrates are recognized. 



Class I. CYCLOSTOMATA (Gr. kyklos, circle; stoma, mouth). - 

 LAMPREYS and HAGS (Figs. 352-356). Cold-blooded, fish-like 

 vertebrates without jaws and lateral fins. 



Class II. ELASMOBRANCHII (Gr. elasmos, metal plate; bran- 

 chia, gills). SHARKS, RAYS, and CHIMERAS (Figs. 358-367). 

 Cold-blooded, fish-like vertebrates with jaws, a cartilaginous 

 skeleton, a persistent notochord, and placoid scales. 



Class III. PISCES (Lat. piscis, fish). FISHES (Figs. 368- 

 408). Cold-blooded vertebrates with jaws, and usually with 

 lateral fins supported by fin-rays. They breathe chiefly by gills. 



Class IV. AMPHIBIA (Gr. amphi, both; bios, life). FROGS, 

 TOADS, and SALAMANDERS (Figs'. 409-438). Cold-blooded, 

 naked vertebrates mostly with pentadactyle (five- fingered) 

 limbs. The young are usually aquatic and breathe by gills; the 

 adults usually lose the gills, and breathe by means of lungs. 



