i] STRUCTURAL AND SYSTEMATIC 31 



acquainted with a few such forms, and even with 

 some which live at will in either fresh, salt, or 

 brackish water. Of these something will be said later. 

 These forms have also been collectively treated of 

 as Microdrili, a term which expresses the undoubted 

 fact that they are all of small size and sometimes 

 even minute ; others however reach the dimensions 

 of the smaller species of earthworms. There are a 

 certain number of characters shared by the various 

 families which may be considered first of all, before 

 dividing them into their several subdivisions. These 

 aquatic Oligochaetes are usually tender and trans- 

 parent, the muscular layers of the body wall being 

 much reduced as compared with the tougher terrestrial 

 forms. The clitellum is also thinner and consists of 

 a single layer of cells only, thus contrasting with 

 the double layered clitellum of earthworms. As a 

 rule the alimentary tract is simplified, there being 

 no gizzard or glandular appendices of the oesophagus 

 comparable to the calciferous glands of most earth- 

 worms. But this rule is not without exceptions ; for 

 we find in Haplotaxis a gizzard occasionally de- 

 veloped, and in the remarkable genus Agriodrilus 

 from the Baikal lake a continuous gizzard formation 

 along the oesophagus, while the Enchytraeidae may 

 show something very like calciferous glands : and 

 even a Tubificid, called by Pierantoni Limnodriloides, 

 has a pair of diverticula of the gut. 



