278 . Dr Erichson on the Classification of 
that irradiation is proportional to the duration of the observa- 
tion, is taken in connection with the observed fact, that the 
volume of the light decreased as the motion of the lenses was 
accelerated, it seems almost impossible to avoid connecting 
together the two phenomena as cause and effect, 
On the Classification of Invertebrate Animals. By Dr W. P. 
ERicuson. 
Ir has, in recent times, been the practice almost universally 
to unite the articulated worms with insects, the two conjoined 
being regarded as forming one single principal division, the 
Articulata; whether this division be considered (as it is by 
Blainville and others) as including all invertebrate animals pos- 
sessing to a certain extent a symmetrical structure, or whether 
(as Ehrenberg, from a more profound study of internal organi- 
sation, proposes) it be limited to animals in which the articula- 
tion of the body is shewn to be a true one, by the existence of a 
nervous system consisting of a row of ganglions with radiating 
nerves. It comes to be a question, however, whether symmetry 
and articulation of the body, and the form of the nervous system 
connected with the latter, indicate so much that is not afforded 
to systematic writers by other considerations. I have at least 
myself arrived at the conviction that we must return to the 
Linnean classification, and, in accordance with nature, divide 
invertebrate animals into two great divisions, of which the 
one would correspond with the Linnzan insects, and the other 
with the Linnzan worms. I shall discuss this in the follow- 
ing remarks. 
The first distinction that strikes us between the two consists 
in this,—that the one group possesses a certain system of organs 
of motion, but the other does not, and, as no passage takes 
place, but, on the contrary, all the Linnean insects, at least 
during a certain period of life, are provided with these, while 
in the Linnean worms there is nothing analogous, this dis- 
.tinction is constant and decided; and, as voluntary motion 
