The Worms and some of their Posterity 
reproductive bodies into the digestive cavity, but directly to the 
exterior by means of a special opening. Each individual has a 
pair of male and a pair of female reproductive glands, but the 
eggs are not self-fertilised; nor is the fertilisation of the eggs 
trusted to chance and the sea-water, as in the lower groups. 
Instead there is a definite exchange of sperms between two in- 
dividuals, and the eggs are fertilised before they leave the body. 
They are also frequently supplied with a store of nutritive material 
by a pair of special yolk glands. A distinct step of progress can 
thus be recognised in the arrangements for reproduction. Between 
the outer skin and the inner digestive layer is developed a con- 
siderable mass of cells, forming muscular and connective tissue, 
etc. It will readily be understood that the development of such 
thick tissue masses occasions two distinct new difficulties in the 
animal economy; for where cells are in direct contact neither 
With the digestive layer nor with the exterior, their nutrition 
and the removal of their waste products can no longer be efficiently 
carried on without special devices. Hence on the one hand a 
circulatory system, for the transport of food materials, and on 
the other an excretory system, become necessary. The first of 
these new departures was not destined to be made until the next 
stage of progress; the Turbellarians seem to have temporarily 
got over the difficulty, like the Ctenophora, by developing a com- 
plex and ramifying digestive cavity. An excretory system, 
however, makes its appearance here. Indeed, the beginnings of 
such a system can be seen in the Ctenophora, in which there are 
small excretory organs opening into the digestive cavity. The 
corresponding organs in the worms, as in all subsequent types, 
open directly to the outside. In the Turbellarians these organs, 
which are termed nephridia, are two in number, and consist of 
long tubes which branch and ramify throughout the body, the 
small branches terminating in special excreting cells, and the 
whole constituting a complete and thorough drainage system. 
The nervous system consists of one or two small masses of nerve 
cells termed ganglia in the front region, with a somewhat complex 
network of nerves connecting them with various parts of the 
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