Benham. — The Oligochaeta of the Nexo Zealand Lakes. 195 



character of the earthworms, it appears somewhat disappoint- 

 ing to find such an absence of pecuHar genera in the case of 

 the aquatic worms. Nevertheless, this is in accordance with 

 the usual character of fresh- water fauna the world over — at 

 any rate, in the case of the smaller invertebrates — and espe- 

 cially of the OligocJiceta, for there is a remarkable uniformit}^ 

 in the aquatic annelids in all parts of the world hitherto 

 explored, with a mere sprinklmg of peculiar forms. And this 

 difference in the facies of the terrestrial and aquatic annelids is 

 due to the difference in the manner in which the two kinds of 

 Oligochceta are transported from one country to another. 



Now, as I have pointed out in previous communications, 

 earthworms can only be distributed by their own active loco- 

 motion or migration through or on the earth, hence con- 

 tinuous land-areas — at some period or another — are necessary 

 to explain the occurrence of allied species in two spots now 

 isolated by stretches of sea. But in the case of the fresh- 

 water worms, which are of small size, the means of dispersal 

 from one country to another does not necessarily depend on 

 their own emigration ; and though in this case we are still 

 somewhat in the dark as to the exact manner of transport, 

 yet from analogy with what we know to be the mode of dis- 

 persal of the smaller aquatic molluscs, Crustacea, and insects 

 it appears extremely probable that these small aquatic worms 

 may be conveyed by birds, especially by wading-birds, such as 

 the red stilt and heron, and by ducks like the teal, grey duck, 

 and crested grebe, which, dabbling in the mud at the margin 

 of a lake or in the neighbouring swamp or stream, carry away 

 some of this mud on their feet. The occurrence, for example, 

 of Plutellus in Wakatipu and of Diporochceta in Manapouri, 

 both of which worms have their home in Australia, is probably 

 to be explained by their being carried, or by their cocoons 

 being carried, in this mud adhering to the feet of such birds. 

 The length of time required by a fairly strong-flying bird with 

 a wind behind it in passing across the Tasman Sea would not 

 be more than thirty-six to forty-eight hours — and perhaps 

 even less — and during this time, if the worm were enclosed in 

 a ball of earth, it is quite possible that sufficient moisture 

 would be retained in the centre of the ball to preserve the life 

 of this small worm. At any rate, the cocoon would easily 

 withstand a considerable amount of drying-up, and when the 

 bird alights on the shore of the lake the cocoon, deposited in 

 the water, would give the species a start in the new locality, 

 and as each cocoon contains several eggs the species would 

 have a fair chance of survival. Since the mud continues to 

 adhere to the feet, it is clear that a certain amount of moisture 

 persists, otherwise the mud would crumble away during the 

 journey. Now, M. De Guerne has investigated the mud ad- 



