VERTEBRATES 



243 



Class I. Gastrop'oda (belly-footed). With or without shells, which are usually of one piece 



and coiled. Snail, whelk, slug. 

 Class II. Pelecyp'oda (hatchet-footed). Shells of two valves or parts. Clams, oysters, 



scallops, mussels, etc. 

 Class III. Cephalop'oda (head-footed). Foot partly surrounds head and bears tentacles or 



grasping organs. Squid, octopus, cuttlefish, etc. 



X. Vertebra'ta. — All the animals we have studied thus far 

 agree in having whatever skeleton or hard parts they possess on 

 the outside of the body. Collectively, they are called Inver'te- 

 brates. In the higher animals called Ver'tebrates, the main or 

 axial skeleton is inside of the body. The exoskeleton of inverte- 

 brates is dead, being secreted by the cells within, while the endo- 

 skeleton of vertebrates is, in part at least, alive and is capable of 

 growth; for example, 

 a broken arm or leg 

 bone will grow to- 

 gether. In man, cer- 

 tain parts of the skele- 

 ton, as nails or hair, 

 are formed by the skin, 

 and in addition, he 

 possesses inside bones 

 to which the muscles 

 are attached. Some of 

 these bones are ar- 

 ranged in a flexible 



column in the dorsal (the back) part of the body. This vertebral 

 column, as it is called, is distinctive of all vertebrates. Within 

 the bony protection of this column lies the delicate spinal cord, 

 and to it are attached the big bones of the legs and arms. 

 The vertebrate animals deserve more of our attention than other 

 forms of life because man himself is a vertebrate. There are 

 37,000 known species of vertebrates. 



There are five groups or classes of vertebrates : Pisces, or Fishes ; Amphib'ia, 

 or Amphibians ; i^ep^i^m, or Reptiles ; A 'yes, or Birds; Mamma 'Zia, or Mam- 

 mals. Let us see how to distinguish one class from another. 



Fishes. — Fishes are familiar animals to most of us. We know that they 

 live in the water, and that they have a backbone and fins. The paired fins are 

 four in number and correspond in position and structure with the paired limbs 

 of man. They are called pectoral and pelvic fins, because they are attached to 



Skeleton of a dog a typical vertebrate. 



