342 C. E Beecher — Origin and Significance of Spines. 



produced a corresponding suppression of appendages on or 

 near the extremity of the abdomen. This statement of fact is 

 the basis of the principle of cephalization of Dana, 1 ' who applies 

 it especially to the Crustacea, as follows : " There is in general, 

 with the rising grade, an abbreviation relatively of the abdomen, 

 an abbreviation also of the cephalothorax and of the antennae 

 and other cephalic organs, and a compacting of the structure 

 before and behind ; a change in the abdomen from an organ 

 of great size and power and chief reliance in locomotion, to 

 one of diminutive size, and no locomotive power." Audouin's 

 law that among the Articulata, one part is developed at the 

 expense of another, may be also noticed here as affording a 

 further explanation of the suppression of the posterior append- 

 ages correlative with the greater development of the parts 

 anterior to them. In a Crustacean using its tail for propulsion, 

 as the Lobster (Romarus), the telson is broad and flat, and the 

 adjacent segment has a similar development of the appendages. 

 In other forms, as the Horse-Shoe Crab (Limulus) and the 

 Phyllocarids, 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 Phyllocarids, show a greater or less suppres- 

 sion of the last abdominal appendages, and in many genera, 

 the body terminates 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 cephalo- 

 thorax, 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 Prestwichia among Limuloids ; 

 and Olenellus among the Trilobites. In addition to a telson 

 spine, the Phyllocarids 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 sup- 

 pression, 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 

 Phyllocarids, 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 



