240 



on one side and three on the other, while the disk looks as if it 

 had been cut in two by a knife, and three new arms had then grown 

 out from the cut side. 



Echinoderms as a rule, then, are reproduced alone by eggs and 

 sperm cells. After fertilization of the egg they pass through : 



1 . Morula stage. 



2. Gastrula stage. 



3. A larval, temporary stage (Pluteus, Brachiolaria, Auricu- 



4. The Eehinoderm grows from a water tube of the larva, 

 finally absorbing the latter, whose form is often materially 

 changed during the process. It thus undergoes a true metamor- 

 phosis, in a degree comparable with that of some insects. 



EEVIEWS AND BOOK NOTICES. 



The Geological Survey ok Missouri. — We have too long de- 

 layed our notice of the two octavo volumes from the geological 

 survey of Missouri, which, though bearing the date of 1873, were 

 not distributed till 1874. The first of these is a collection of re- 

 ports from 1855 to 1871, by Messrs. Brodhead, Meek & Sbumari, 

 and the second, the results of the work of 1872, is devoted to tlie 

 iron and coal deposits of the state. Of these the former are de- 

 scribed by Dr. Adolph Schmidt, and the latter by Mr. Brodhead ; 

 in addition to which the late director, Prof. Raphael Pumpelly,has 

 prefixed an important chapter on the geology of the Pilot Knob 

 district, and its iron ores, from which, and from the copious de- 

 scriptions of Dr. Schmidt, we gather a pretty complete account of 

 this extremely curious region. Rising above the floor of horizon- 

 tal paheozoic deposits, the 3.1 Magnesian limestone of Swallow, a 

 member of a group of strata supposed to correspond to the Pots- 

 dam of New York, appear numerous hills of crystalline rock, de- 

 scribed as exposed portions of the skeleton of the eastern part of 



