126 How Animals Changed 



vided with stout spines which hold fast when an animal runs on 

 land. Arthropods appear to be related to annelids. In fact, if some 

 polychaete annelids were to be provided with an exoskeleton, they 

 would become arthropods without further modification. Harms 

 (1929) has described a tough-skinned polychaete which walks about 

 on land by using its parapodia somewhat as a centipede uses its 

 legs. Land crustaceans have stouter, more spiny, and less setose 

 limbs than their aquatic relatives; but a strictly aquatic crab which 

 is suddenly placed on land is not unsuited to locomotion and, if 

 permitted, quickly makes its way back to water. Crabs show various 

 adaptations in foot structure for digging, clinging, etc. (Crane, 

 1947). The locomotor difficulties of arthropods in attaining land 

 life have been few and simple. 



The origin of the tetrapod limbs of vertebrates is more or less of 

 a mystery, but paleontologists and comparative anatomists rather 

 generally agree that they came from lobe-fins such as crossoptery- 

 gian ganoids possessed in Devonian times. "The tendency toward 

 the evolution of freely turning paddles, presumably out of fin-folds, 

 reaches a climax among the lobe-finned fishes of late Paleozoic times 

 ... the paddles spread out like the sticks of a fan, and the bony 

 rods that support them seem destined to give rise to the skeleton of 

 the arm and hand in higher vertebrates" (Gregory, 1933) . "At all 

 events it is clear that we should expect the fish-like ancestors of the 

 Tetrapoda to have possessed pectoral and pelvic fins alike in struc- 

 ture, with outstanding muscular lobe, extensive endoskeleton with 

 at least five rddials, small web, and few if any dermal rays" (Good- 

 rich, 1930). Lull (1929) believes that the earliest tetrapod limbs 

 may have been two-toed but agrees with others that the majority of 

 primitive land vertebrates were five-toed. 



The limbs of certain modern fishes which live out of water on 

 muddy beaches have been described in considerable detail (Ham- 

 burger, 1904; Eggert, 1929a) . The ventral, or posterior, fins of 

 gobies often together form sucker-like organs which are used for 



