THE ANIMAL CLIMAX-THE CHORDATES 



285 



of the animal to the other. The muscles are 

 segmentally arranged and by their rhyth- 

 mic contractions make possible lateral un- 

 dulations of the body used in swimming. 

 Immediately above the notochord is the 

 hollow nerve cord which tenninates anteri- 

 orly in a light-sensitive end organ, the eye- 

 spot. Numerous gills function in breathing. 

 Its digestive and circulatory systems are 

 relatively simple and add nothing to what 

 we have already seen among the inverte- 

 brates. In fundamental plan, however, these 

 organ systems, like so many other features 

 of this little animal, show great similarity 

 to the vertebrates and thus point to the pos- 

 sibility that the vertebrates may well have 

 come from a form not greatly unlike it. 



THE FIRST ANIMALS WITH 

 BACKBONES-VERTEBRATES 



It was not until the chordates somehow 

 acquired a rigid internal skeleton that they 

 became important. There were other fac- 

 tors, of course, but certainly if the group 

 was to advance it needed a substantial in- 

 ternal support upon which a body could 

 be built that would succeed not only in an 

 aquatic environment but also on land. This 

 was accomplished in the development of 

 the vertebral column, or backbone. Let us 

 examine a few typical examples of this 

 highly successful group of the vertebrates. 



The first members of the sub-phylum 

 Vertebrata that show the beginnings of a 

 backbone are the cyclostomes, the "round- 

 mouthed" eels (class Cyclostomata — 

 round mouth). Typical representatives of 

 this class are the lampreys. They have no 

 appendages and no jaws, only a circular 

 mouth lined with denticles, small tooth-like 

 structures that aid in clinging to prey. 

 When the lamprey seizes a bony fish, its 

 usual prey, it first attaches itself with the 

 sucking mouth and then proceeds to re- 

 move small bits of tissue with its rasping 

 tongue. If the point of attachment happens 

 to be in the abdominal region a perforation 



Fig. 13-7. Amphioxus (Branc/iiosfoma californiensis), 

 partly emerged from its burrow along the California 

 coast. 



is made through the body wall and the 

 internal organs injured so severely that 

 the fish usually dies shortly. However, if the 

 injury occurs on the dorsal side over the 

 large muscles, the effect is usually not fatal. 

 The common sea lamprey, Petromyzon, has 

 invaded rivers and streams where it has 

 become a formidable foe of fish populations 

 (Fig. 13-9). These ravages have been par- 

 ticularly severe in the Great Lakes region 

 where in many areas commercial fishing has 

 all but ceased on this account. Efforts are 

 being undertaken to destroy them during 

 their nesting period, which takes place in 

 small streams. Thus far, however, little prog- 

 ress has been made against them. 



Internally as well as externally the lam- 

 prey shows its lowly origin (Fig. 13-10). It 

 retains a notochord similar to amphioxus, 

 but also has the beginnings of a spinal 

 column and other internal skeletal parts 

 which, however, are composed of cartilage. 

 There is a well-developed brain, together 

 with an olfactory organ, a pair of poorly 



