While there are several kinds of earthworms that are thus met with in fresh water, others will live for some time submerged {367}in water. Perrier 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 _Phreoryctes_, 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 _Branchiura_ 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 progression through dry soil; and it is to be noted that those Oligochaeta, 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 {368}integumental blood capillaries. These are indeed for the most part absent in the aquatic forms, being only faintly developed in a few, an example possibly of degeneration.