69 



Notes on the Marine Oligochaeta of Plymouth. 



By 



Frank E. Beddard, H.A., 



Prosector to the Zoological Society of London ; Lecturer on Biology at Guy's Hospital. 



The Oligocliaeta form a division of the Annelida, of "wHcli the 

 most familiar type is the common earthworm {Lumhricus terrestris) ; 

 the group comprises also a great number of smaller worms, which 

 are for the most part inhabitants of ponds and streams, such as the 

 red River worm [Tuhifex riviijorum). The Oligochgeta were at one 

 time believed to be entirely terrestrial or inhabitants of fresh water, 

 and to be distinguished thus from the Polychgeta, which were 

 supposed to be exclusively marine in their habitat. Although the 

 progress of research has not broken down the structural distinctions 

 between these two divisions of the Annelida chsstopoda, it has been 

 proved that no absolute line of demarcation can be drawn between 

 the Oligochseta and the Polychasta as regards their habitat ; on the 

 one hand Polychseta have been found in fresh water, and, on the 

 other hand, certain species of Oligochaeta are now known to inhabit 

 the mud and gravel of the seashore. 



There are three species common in the Sound at Plymouth, which 

 are apparently identical with certain species described by Claparede* 

 from the shores of Scotland and France. 



One of these is a small whitish worm about half an inch in 

 length, which is abundant in gravel between tide marks in Rum 

 Bay. It belongs to the genus Pachydrilus of Claparede, but I have 

 not yet satisfactorily determined to which of the several known 

 species of the genus it should be referred. 



The two remaining species are found in mud at Drake's Island and 

 on the shores of the Sound, where they are extremely abundant. 

 They can be distinguished from each other by a slight differ- 

 ence in the transparency of the body wall. One worm is much 

 more transparent than the other, and is therefore of a brighter 

 red colour, this colour being, of course, due to the blood-vessels ; this 

 species appears to be Claparede's Clitellio arenarius. The other 

 worm has a more dusky appearance, caused by the presence of a 



* Mem. Soc. Phys. Geneve, 1862. 



