CHAPTER L 

 SUBPHYLUM VERTEBRATA 



The last subphylum, Vertebrata, is distinguished from the other 

 subphyla of Chordata by several characteristics, though it shares with 

 them all the characteristics that belong to the chordates as a whole. 

 Instead of a type being selected to illustrate this subphylum, a general 

 description will be given applicable to vertebrates generally. 



343. Distinguishing Characteristics. — Some of the distinguishing 

 characteristics of the vertebrates are as follows: (1) The notochord is 

 more or less completely replaced by a vertebral column, which is made up 

 of a series of separate bones called vertebrae. (2) The vertebral column 

 and other supporting structures form an internal skeleton, or e7ido skeleton. 



(3) As a rule two pairs of ap-pendages, either fins or limbs, are developed. 



(4) All vertebrates possess a heart, ventrally situated, with at least two 

 chambers. (5) In the blood are red blood corpuscles which contain hemo- 

 globin; white blood corpuscles also exist, as they do in the blood of the 

 higher invertebrates, but the hemoglobin was in those types dissolved in 

 the plasma of the blood. (6) All vertebrates possess a brazji which is 

 divided into five parts known as vesicles, each in its primitive form con- 

 taining a cavity. (7) They possess a more or less distinct head, in which 

 are situated several organs of special sense. (8) A large coelom is present 

 which is almost entirely filled with the organs of the digestive, respiratory, 

 excretory, and reproductive systems; the small space not occupied by 

 these systems contains a serous, or watery, fluid. (9) The vertebrates 

 are also alike generally in that they possess a posterior prolongation of 

 the body behind the anal opening, forming a tail. 



344. Body Plan. — Generally speaking, the vertebrate body is divided 

 into three parts — head, trunk, and tail. A neck, which is simply a con- 

 stricted region between the head and trunk, may be present, though 

 this is not marked in the lower vertebrates or those fitted for aquatic 

 life. In the terrestrial types, one of the two pairs of appendages is 

 situated in the thoracic region, which is the anterior portion of the trunk, 

 while the other pair is situated in the pelvic region, which is the posterior 

 portion just in front of the tail. These positions are dictated by mechan- 

 ical necessity when the limbs are used for both support and locomotion. 

 In aquatic types where the body is buoyed up by the water this arrange- 

 ment is frequently modified. The anterior appendages appear in 

 various animals as pectoral fins, forelegs, arms, or wings; the posterior 

 ones are either pelvic fins, hind legs, or the only legs. 



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