THE ACQUIREMENT OF CHARACTERS ILLUSTRATED, l'^^^ 



guishing it from matter in an inorganic state, were differenti- 

 ated before the Cambrian. 



B. Animal . — All the characters distinguishing it from 

 plants. 



C. Molluscoidca. — The special characters of this branch 

 were fully differentiated in the Cambrian era. These are 

 (to follow Claus and Sedgwick): animals attached, as distin- 

 guished from moving or perambulatory organisms; the de- 

 velopment of bilateral symmetry; the absence of metameric 

 division — they are unsegmented; the differentiation of cili- 

 ated oral appendages; enclosure in a calcareous shell, with 

 differentiation of organs into the various physiological 

 systems of the Metazoa, viz., digestive, motor, neural, ex- 

 cretory, and those of reproduction. 



D. Class BracJiiopoda (and not Polyzoa). — This distinc- 

 tion includes the characters of two spirally rolled buccal 

 arms; the development of bivalve, equilateral, dorsal, and 

 ventral shells; the development of several ganglia connected 

 by a pharyngeal ring. There must be included here also all 

 the characters which are necessary to carrying on the func- 

 tions of the different parts, mentioned under groups C and D. 



E. Lyopomata and ArtJiroponiata. — All that distinguishes 

 these two orders was fully evolved, certainly, before the 

 Cambrian era was far advanced, for we find several dis- 

 tinct families of the one and five of the other already in the 

 Cambrian rocks. These differences are seen by comparing 

 specimens of Tcrcbratnlina with a Lingiila — both recent 

 genera. The differentiation includes, in respect of intestine, 

 a long and open intestine, with anal as well as oral orifice, 

 and short, with postero-ventral end closed; in the shells the 

 distinction between free-sliding valves and hinged, articulated 

 valves, and the associated modification of muscular apparatus 

 to move them laterally upon each other in the first case, and 

 to open and shut them with a hinge in the other. 



Perpetuation and Repetition of Characters a Common Law of 

 Generation. — The more sharply distinguishable characters are 

 mentioned above, but they include more than the ordinary 

 observ^er would notice if handed a specimen and asked to 

 describe what he saw — more, I say, but not all the characters 



