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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