84 STUDIES IN EVOLUTION 



development of the appendages. In other forms, as the 

 Horse-shoe Crab (Limulus) and the Phyllocarida, the tail 

 is not used for propulsion, and at best serves chiefly as a 

 rudder, while some of the legs on the anterior part of the 

 abdomen or on the thorax are large and strong and are often 

 provided with paddles. These groups, the limuloids and 

 Phyllocarida, show a greater or less suppression of the last 

 abdominal appendages, and in many genera the body termi- 

 nates in a spiniform telson or tail spine. The process of 

 suppression may or may not result in a spine. In the crabs 

 the abbreviated abdomen is folded under the cephalothorax, 

 and in Lepidurus and Pterygotus the telson is a scale or 

 plate-like organ. For the most part, however, the abbrevia- 

 tion of the abdomen and the suppression of its appendages 

 have reduced the telson to a spine; as in Limulus (figure 67), 

 Eurypterus, Stylonurus, and PrestwicTiia among limuloids, 

 and Olenellus among the Trilobita. In addition to a telson 

 spine, the Phyllocarida have two lateral spiniform cercopods, 

 the three spines together constituting the post-abdomen ; as 

 in Ceratiocaris, Echinocaris (figure 68), Mesothyra, etc. 



Although the last abdominal segments of the Horse-shoe 

 Crab have lost their appendages and show evidences of 

 suppression, yet the tail spine is a large and useful organ, 

 for it is of just the proper length to enable the animal to 

 right itself after being overturned, which it is unable to 

 do with its feet alone. The process of natural selection has 

 doubtless in this way contributed to the development and 

 retention of the long spine. This use cannot be ascribed 

 to the tail spines of the Phyllocarida, though they evidently 

 were important aids in directing movement, and also offered 

 some degree of protection. 



The terminal claws on the phalanges of the wings of some 

 birds are nearly all that remain of the external fingers or 

 digits. In the Hoactzin of South America (Opisthocomus 

 cristatus) the young bird has a thumb and index finger, both 

 provided with claws, and climbs about much like a quad- 

 ruped, using its feet, fingered wings, and beak. According 



