Invertebrate Animals. 281 
jaws of insects are actually identical with the legs, as is evident 
from their conversion into legs (asin the Arachnidee), and, vice 
versa, from the conversion of legs into jaws (as in the true Crus- 
tacea), the assumption that the head of insects, like the thorax, 
is composed of three segments, can in no way be made out. 
The constant occurrence of three pairs of jaws, however, ad- 
mits of the supposition that the head, even where it is appa- 
rently awanting, inasmuch as it is blended with the imme- 
diately following sections of the body,—as in the Arachnide, 
the Decapods, the Stomapods, and the Entomostraca,—is 
nevertheless an essential portion of the body of insects. This 
_ also may be deduced from the constant presence of the brain 
as a central organ of the nervous system, even where, exter- 
nally, a determinate well-marked head is awanting. 
In the Linnean worms (Vermes), on the other hand, a head 
is not present, in the sense in which it is in the whole verte- 
brate animals, in all insects in the more confined signification, 
and in the greater number of Crustacea, even in the Myria- 
pods, in which the further division into the larger subdivi- 
sions of the body is the least developed; and I cannot con- 
vince myself that Linnzus was so far wrong as he is thought 
to be by many distinguished naturalists, when he denies that 
the Vermes possess a head. A head, strictly speaking, can 
only be considered in contradistinction to the trunk. This 
distinction does not occur in the Linnzan worms, where the 
whole body is nothing else but a living abdomen, where, even 
when the body is articulated, no other distinctions prevail be- 
tween the individual subdivisions than sometimes the ramifi- 
cation of the circulating system, and where, lastly, at the same 
time, the anterior portion is distinguished by the cirriand organs 
of sense (viz. the eyes) placed there, and this only because the 
mouth-opening is there situated. Thus, even in the Cepha- 
lopods, the anterior portion of the body not included by the 
mantle, presents itself not as an actual head, and so much the 
less because this part of the body is turned downwards, and is 
that on which the animal creeps; and still less can the same 
portion claim such a title in the Pteropods, inasmuch as in that 
group the eyes are not even placed there. In the Annelides, 
it is true, the first segment of the body is frequently distin- 
