VALUE AND USE OF STAGES IN DEVELOPMENT o99 



the foiled ('o|)lialoiH)ila, whoiv l),v bivaking hack the shell one can get 

 youthful eharaeters very perfectly preserved at the apical portion of the 

 test, which has been ])r()tei'ted from wear by the involute coiling. 



Studies which Beecher made on the development ol' the Trilobita 

 opened up our knowledge of the group so as to make it a new thing. 

 Here, as' the animal molted the test at each period of growth, we have 

 to get the actual young and individuals preserved at just the right period 

 of growth in order to obtain successive stages in development. In the 

 early i)rotaspis stage the body consists of a ccphalon and pygidium, with 

 no thoracic segments and, except in accelerated types, with no eyes or 

 live cheeks visible from the dorsal side. Later, thoracic segments appear, 

 one at a time, witli successive molts, the eyes and free checks travel in, 

 ornamentation and other characters a])pear ])rogrcssively until full species 

 features are attained. Primitive trilobites (Ptychoparia, Sao) show a 

 relatively slow development and specialized types (Acidaspis, Arges) an 

 accelerated development. This same principle holds widely with primi- 

 tive types, which retain stages in development with remarkable pertinac- 

 ity. On the other hand, specialized types have such quick development 

 that stages are often run together or telescoped, or even may be skipped 

 altogether. As a result of his beautiful studies, Becchcr made out a 

 classification of the Trilobita based on ontogeny — a great achievement to 

 attain in so difficult a group. 



Stages in development as applied to fossil animals, or, better still, to 

 a combined study of fossil and recent, open up a field of research which 

 is almost unlimited, and the work that has been done in this line, espe- 

 cially by the Hyatt school of investigators, shows that this method of 

 study throws great light on the important lines of anatomy, morphology, 

 and phylogeny. 



