LIFE IN PRIMARY PERIOD 219 



passage of large Worms may be responsible for tracks such 

 as those that have been grouped as Biiobites, and that have 

 been attributed occasionally to Trilobites. To the Worms must 

 be linked other animal organisms, the Brachiopods, which 

 were long taken for Molluscs, but are still isolated. Morse 

 has given a very interesting explanation of these organisms, 

 which has inspired his theory of cefihalization. He draws 

 attention to the fact that in the majority of segmented 

 organisms the part nearest the mouth undergoes greater 

 development proportionately than the posterior region, which 

 tends to diminish and disappear. Among the Merostomata 

 and Trilobites, the body ends in a point ; that of the Scorpions 

 has shrunk to a post-abdomen bearing the poison-sting. In 

 the majority of the Batrachians, Reptiles, and Mammals, the 

 viscera are so much concentrated in the anterior or at least 

 medial region that the posterior part of the body becomes 

 a tail behind the anus, which tail is sometimes used for pre- 

 hension, but may disappear altogether when not used. This 

 phenomenon is easily explained on Lamarck's principle. In 

 the anterior region of the body are assembled together not 

 only the mouth but the sense organs. This portion initiates 

 the movements which drag the rest of the body, and the 

 posterior region can but follow. The anterior region therefore 

 is generally the active region par excellence, and the one which, 

 according to the principle we have just invoked, ought to 

 attain the maximum development, while the inactive part is 

 atrophied. That is the reason why the number of body 

 segments in the Arthropods and the higher Worms tends to 

 be reduced to a fixed, indispensable minimum. If, however, 

 the posterior region of the body does become active, after 

 this reduction has taken place, the segments will not increase 

 in number, but they will be very large. The higher Crustaceans 

 which swim, like Squilla or the Crayfish, by striking the 

 water with a suddenly flexed abdomen, have this part power- 

 fully developed, while in the Crabs, essentially walking 

 creatures, the abdomen is atrophied. In the same way Fishes, 

 Cetaceans and Sirenians, which swim by striking the water 

 laterally with their tail, possess broad tails. The marine 

 tubicolous Worms, which bury themselves in the mud, live 

 under conditions which favour the marked development of 

 the anterior part of their body at the expense of the posterior 



