170 BRODE. [VoL. XIV. 
In illustration I may cite the case of Microstoma. This is a 
small Turbellarian which multiplies by means of fission. This 
unsegmented worm possesses an alimentary canal extending 
the length of the body, a so-called “brain” with two lateral 
nerve trunks and well-marked sense organs situated at the 
anterior end of the animal. 
-The process of fission is such that the animal may show, 
according to von Graff, as many as sixteen individuals of vary- 
ing ages in one chain. These subsequently separate, forming 
so many complete individuals. Before separation occurs we 
have a chain of individuals with a common alimentary canal and 
a nervous system extending the whole length of the chain. 
Each individual shows one or two pairs of sense organs at its 
anterior end. The individual mouth openings have not as yet 
pushed through. Should this temporary condition become 
permanent, we should have a segmented form resembling in 
some essential features an annelid worm. 
In the different forms of animals in which fission occurs we 
find several modes of fission. In the form mentioned each 
individual proliferates continuously. In some forms the pro- 
liferation is confined to one individual, while in others each 
individual in turn takes part in the proliferation. This last 
mode could be applied to the segmentation of an annelid. 
Supposing that annelids arose in this manner, we should 
expect to find the segments of the body practically homodyna- 
mous, with the more perfectly developed and highly specialized 
segments at the anterior end of the worm. 
In Dero, as in many other forms, there is in the adult form 
a marked “cephalization.”’ Apparently the first five segments 
are formed after the trunk segments are laid down. A study 
of the embryological development of the worm may clear away 
this apparent objection to the theory, as Professor Whitman 
has shown in the case of Clepsine, that, while differentiation 
does not become apparent in the head as early as it does in the 
trunk, the segments forming the head are really laid down 
before those of the trunk region. 
In support of the theory I have shown that in Dero there is 
a ganglionic swelling and four lateral nerves for each segment 
