26 NOMENCLATURE 
and Enaima of Aristotle those of Myeloneura 
and Ganglioneura. 
But it is not my object to give all the classifica- 
tions of different authors here, and I will there- 
fore pass over many noted ones, as those of Bur- 
meister, Milne-Edwards, Siebold and Stannius, 
Owen, Leuckart, Vogt, Van Beneden, and others, 
and proceed to give some account of one investi- 
gator who did as much for the progress of Zodlogy 
as Cuvier, though he is comparatively little known 
among us. 
Karl Ernst von Baer proposed a classification 
based, like Cuvier’s, upon plan; but he recognized 
what Cuvier failed to perceive,—namely, the 
importance of distinguishing between type (by 
which he means exactly what Cuvier means by 
plan) and complication of structure,—in other 
words, between plan and the execution of the 
plan. He recognized four types, which corre- 
spond exactly to Cuvier’s four plans, though he 
calls them by different names. Let us compare 
them. 
Cuvier. Baer. 
Radiates, Peripheric, 
Mollusks, Massive, 
Articulates, Longitudinal, 
Vertebrates. Doubly Symmetrical. 
Though perhaps less felicitous, the names of Baer 
express the same ideas as those of Cuvier. By 
the Peripheric type he signified those animals in 
