8 J. D. Dana on Megasthenes and Microsthenes. 
form or habit of body (some Crabs are called sea-spiders) ; and 
(2) in the coalescence of the thoracic and abdominal nervous 
ganglions into a single central thoracic ganglion. At the same 
time, the division of Scorpions, among Spiders, is fag aeg toe. 
related to that of the Macrural Decapods, (1) in the body con- 
sisting of a series of segments; and (2) in the crs ganglions 
being neler one to each abdominal segment. Moreover the 
ds are ae and chelate, ik the outer pair in some in- 
forior I acruran 
ain, the Mynipede are distantly related to the Tetradecapods, 
they being similar in their annulated structure, each segment 
having its pair of feet, and some species of the former (as those 
of Glomeris) even resembling the nese quite closely in form, 
articulation, and antennz, and many of them having also the 
habit of some Oniscidce (Tetrsdecapods) of rolling into a ball. 
Thus, the second order of Insecteans is related, as regards form, 
to the first of Crustaceans; and the third of Insecteans, to the 
second of Crustaceans. 
The earliest of Crustaceans, the TJrilobites, one of the compre 
hensive types as styled_by the writer, are, ‘therefore, not only 
intermediate between Entomostracans and Tetradecapods, but 
also, in some respects, between these and the Myriapods. 
nes like the latter, Trilobites are abnormal in the very large 
of segments of which the body is composed; and some 
times at pe P present no distinction between the ‘cephalotho- 
a 
“tthe facts pointed out prove conclusively that Insecteans and 
rustaceans constitute classes of equivalent value. 
2. Megasthenes and Microsthenes. 
The two grand divisions of typical brute geen the Meg- 
asthenes and Microsthenes, are not separated by any very marked 
difference in type of structure; and still there is a profound fun- 
damental difference between them »—that, to which the names 
- This is in contrast with the fact among Crustaceans, the 
i and microsthenic divisions of which (the Decapods 
d Tetradecapods) stand widel ely apart. But in the class of Crus 
nag oaks limited range of variation. Hence, in the distinctions 
of Megasthenes and Microsthenes, among Mammals, we cannot 
look ‘he the marked diversity that subsists between Decapods 
and Tetradecapods, although the naturalness of the subdivisions 
is none the less real. bed Ph egencephals and Micrencep 
signifying large-brained small-brained Mammals) may better _ 
sty ” the desire for rhe expressing something tangible in 
ucture. Yet they do not appear to indicate the fanda- 
mona stinction between the groups. A general structural — 
