CHAPTER XIX 

 THE ANNELIDA 



THE metamerically segmented worms are grouped together 

 in a phylum Annelida, the name having reference to the 

 external annulation of their bodies. The phylum comprises 

 four classes, the Archiannelida, the Chgetopoda, or bristle 

 worms, the Hirudinea or leeches, and the Echiuroidea. 



As an example of the Archiannelida we may take a small 

 marine worm called Polygordius, which is tolerably common 

 in the sand at a few fathoms depth in the Mediterranean Sea. 

 It is a small thread-like animal, only some three or four 

 centimetres in length and is externally marked into segments 

 by indistinct grooves. At its anterior end there is a distinct 

 head-lobe or prostomium which bears a pair of tentacles. 

 The mouth is situated on the ventral surface of the segment 

 following the prostomium, and the anus is placed on a terminal 

 swollen anal segment. On each side of the prostomium there 

 is a small oval depression or pit lined with cilia, and probably 

 functioning as a sense organ. There are no chaetae nor any 

 other appendages besides the prostomial tentacles. 



The internal anatomy follows the same plan as that of 

 Lumbricus, but is simpler and more primitive. The nervous 

 system consists of a ventral nerve cord connected by cords 

 passing round the gullet with a cephalic ganglion which lies in 

 the prostomium. (In the earthworm the cephalic or supra- 

 pharyngeal ganglion has been shifted back to the 3rd seg- 

 ment.) The intestine is a simple straight tube passing from 

 mouth to anus and separated by the coelomic cavity from the 

 body-wall. The ccelom is divided into compartments by 

 septa, and the internal segmentation corresponds with the 

 external segmentation. But whereas in the earthworm the 

 right and left coelomic cavities in each somite are confluent 

 dorsally and ventrally, in Polygordius they are separated by 

 vertical partitions, and so the gut is suspended in the ccelom 



45 



