xni EFFECTS OF SEA-WATER HABITAT 367 



in water. Terrier found by experiment that various species 

 could undergo with impunity a prolonged immersion in water, 

 and I confirmed his experiments myself with a common species 

 of Allolobophora. A correspondent of " Nature " stated that a 

 certain number of species (not particularised) of earthworms in 

 Ceylon could suffer with impunity the effects of sea-water. The 

 importance of this fact will be again dealt with in considering 

 the geographical distribution of the group. 



Among the aquatic genera of Oligochaeta we do not as a 

 rule meet with amphibious species. The Enchytraeidae however, 

 as already mentioned, are an exception ; so too appears to be 

 the genus Pkreoryctes, which in its structure is to some extent 

 intermediate between the earthworms and the aquatic families. 



Terrestrial and Aquatic Forms. There are many obvious 

 structural peculiarities which would prevent the normally aquatic 

 worms from being thoroughly at home on dry land. The gills 

 of Brancliiura and the other gilled species would be injured, in 

 all probability, by friction with the earth ; the delicate and long 

 chaetae of Naids and Tubifex are also most unsuited for progres- 

 sion through dry soil ; and it is to be noted that those Oligo- 

 chaeta, which, belonging to aquatic groups, are yet found away 

 from water, have chaetae of the simple sigmoid pattern which 

 characterises the earthworms. 



There are other peculiarities found only in the aquatic species 

 which have not so obvious a relation to their habitat. In no 

 genus that is mainly aquatic in habit are the ova small and 

 nearly unprovided with yolk as in Lumbricus ; the ova of aquatic 

 forms are invariably large and filled with abundant yolk. 



The more delicate organisation of the aquatic Oligochaeta is 

 not so hard to understand. The comparatively unresisting 

 nature of the medium in which they live, water or fine mud, 

 does not necessitate so strong a development of the layers of the 

 body-wall as is essential to the earth-living forms, which have 

 also thick septa in the anterior region, to protect the organs of 

 reproduction as the strong muscular contractions of the body force 

 the worm's way through the dense soil With the weak structure 

 of the integument are perhaps also correlated the simplicity of 

 other organs of the body in the aquatic Oligochaeta. With thin 

 body- walls, through which gases can diffuse with great ease, there 

 would seem to be less need for the development of a system of 



