( ieneral Features 



I Pectora 



t-t-M0UTH->4-« — PHARYNX 



J~ Girdle/; 

 (Appendage; 



-Cloaca 

 **-Cloacal opening 

 , Br -J\Anus 



r, : I p»! ;S\ appendage 



Pancreas i Pelvic^- 1 - 



Pleuroperitoneal 

 coelom 



(Girdle 



-STOMACH-H-* - 



INTESTINE- 



REGIONS OF DIGESTIVE TUBE 



Fig. 10. Diagrammatic sagittal section of a vertebrate. 



rower tail, which is massive and powerfully muscular, and serves as the 

 essential locomotor organ. In mammals the tail is relatively slender and 

 weak. It serves various minor functions, including even expression of 

 emotion in dogs and cats. From the human point of view, it seems more 

 ornamental than useful. It is vestigial or lacking in adult frogs and 

 toads, modern birds, some rodents and bats, and some primates (the 

 group including man, apes, and monkeys). 



Locomotor Appendages 



Among invertebrates, locomotor appendages are highly variable in 

 number. In the one phylum, Arthropoda, are decapod crustaceans, 

 hexapod insects, and centipedes. Modern vertebrates have never more 

 than two pairs. One pair, pectoral, is at the anterior extremity of the 

 trunk; the posterior pair, pelvic, is at the posterior extremity of the 

 trunk. The Paleozoic somewhat sharklike Acanthodii had several (up 

 to five) pairs of relatively small fins between the pectorals and the 

 pelvics (Fig. 320). The pectoral appendages may be fins, legs, arms, or 

 wings. The pelvic pair may be fins or legs. Either pair may be vestigial 

 or lacking — in whales, sea cows, and some bony fishes, the pelvic pair; 

 in birds of the ostrich sort, the pectoral pair : in snakes, "legless lizards," 

 and certain amphibians (caecilians), both pairs. The "round-mouthed" 

 eels (cyclostomes) have no paired appendages and no vestiges of them. 



Most aquatic vertebrates have, in addition to paired fins, median 

 fins which are functional in connection with locomotion. They are 

 highly variable in number and position and are in no way represented 

 in land vertebrates. 



Integument 



Most invertebrates have a skin consisting of a cellular layer, usually 

 only one cell in thickness, which secretes a substance — chitinous as in 



