CHAPTEE IV 



THE ORGANISATION AND DEVELOPMENT OF THE WORMS 



(VERMES) 



THE race of the worms is, even after the exclusion of the Platodes, which 

 till now have been included in it, by no means a natural, well-demarc- 

 ated division of the animal kingdom ; now, as heretofore, it is like a 

 lumber room, to which all those groups are relegated which cannot be 

 placed elsewhere. It is therefore difficult to characterise the race of 

 the worms in other than negative terms. All worms are bilaterally 

 symmetrical animals ; their detailed structure, however, is most varied. 

 They are raised above the Ccelenterata and Platodes by the possession 

 of an anus, and of a blood-vaseular system which undertakes 

 physiologically one of the functions of the gastro-canal system of these 

 animals. Where these systems are wanting a secondary degeneration 

 has perhaps taken place. The mouth lies at the extreme anterior 

 end of the body, originally always on the ventral side. A body 

 cavity is either wanting, or is developed in varying degrees. Under 

 the outer body epithelium there is found in all unshelled forms a 

 generally strong muscular layer (dermo - muscular tube). The 

 nervous system is developed in very different ways. The only 

 constant point is the presence of a nerve centre placed above the 

 oesophagus (brain, supra -cesophageal ganglion). There is also 

 generally a nerve ring surrounding the oesophagus, the cesophageal 

 ring 1 , from which longitudinal trunks run backwards in varying 

 number, position, and arrangement. All these portions brain, 

 cesophageal ring, and longitudinal nerves belong to the central 

 nervous system. Excretory organs (nephridia) are found in all 

 divisions, but under the most different conditions. They often perform 

 the function of conducting the sexual products out of the body. 

 Segmented body appendages (extremities) are as completely wanting 

 as is a specialised muscular organ of locomotion placed on the ventral 

 side (foot). A strictly localised central organ of the blood-vascular 

 system (heart) has been observed only in the Brachiopoda. 



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