Earthworms and Their Allies

by Frank E. Beddard

It is in fact clear enough that whatever may prove to be the case with regard to particular species, a mountain range is not necessarily a barrier to the dispersal of generic types.

The Ocean As a Barrier To Migration.

It is very possible that further investigations into the Oligochaeta will prove that there are more marine forms than those which are enumerated in another chapter. Particularly is this likely to be the case among the family Tubificidae and Naididae. For up to the present those forms belonging to those families which are known to be positively marine in their habit show no great difference from allies inhabiting fresh water, and are in one case indeed (Paranais) common to fresh brackish and saline waters. As to earthworms, the number is also extremely limited, and Pontodrilus is up to the present the only genus which is known to inhabit a marine situation almost exclusively. It has, moreover, been shown that both earthworms and their cocoons are susceptible to salt water and are killed thereby. Thus the facilities which these animals possess of crossing tracts of ocean are limited by this fact alone, besides other impediments offered by tracts of water as such. We may in fact entirely discount the possibility of earthworms floating across arms of the sea—of any extent at any rate. For they do not swim or float, but sink in water. Possibly when the alimentary tract was entirely empty of earth the worms might float; but it is always full and even if evacuated during their passage to the bottom waters the body thus freed would hardly rise. However the noxious qualities of sea water to earthworms is a sufficient barrier to their traversing even narrow straits. On the other hand it might be suggested that torn up trees especially with the roots and clinging earth still attached might harbour worms and thus transmit them to foreign shores. It has been suggested that in this or in some similar way the species of Notiodrilus have been wafted from shore to shore of those lands which are washed by the Antarctic Ocean. Dr Benham, however, in criticising this, calls attention to the violent gales and disturbances of the ocean surface which are so prevalent in those stormy regions, and doubts much whether these animals could retain a safe hold upon some travelling tree trunk. Moreover it is only in this antarctic region where the earthworm fauna of the various continents and islands are so very similar.

Facilities of Migration.

The above brief account of physical features which affect the range in space of the terrestrial Oligochaeta seem to show that the only really important barrier is the ocean; and even a narrow tract of sea water would, as it appears, act fatally in preventing the successful immigration of a race inhabiting one shore to the opposite shore. On the other hand we do undoubtedly find in different countries—even when separated by a large expanse of ocean—closely related forms. The most striking instance of this is that afforded by a consideration of the antarctic species of Notiodrilus and Chilota. Can this interchange of Oligochaetous faunas be explained by any means which earthworms possess of crossing tracts of sea by the aid of living carriers such as birds? It has been definitely shown that these creatures actually do convey such small animals as Mollusca attached to their feet. Is anything of the kind likely in the case of earthworms? In the first place it may be safely asserted that if it be possible it has not been actually proved. This however might be perhaps put down to the lack of sufficient observation of actual birds and the contents of such masses of soil as are found attached to their feet. A consideration of the habits of earthworms seems to imply that such a mode of transference from country to country is unlikely. In the first place we remark that the general behaviour of earthworms renders this unlikely. Even the smaller kinds, whose bulk would allow of their being carried, are too active in their habits to permit of a safe transference. When disturbed they wriggle and progress with activity. It is not conceivable that they would remain quiescent for sufficient time to allow of a long voyage. But while the bodily transference of adult earthworms seems highly improbable it is conceivable at the first view that their cocoons might be so transferred. We require to know rather more about the cocoons of earthworms before we can accept this view as a possibility; as far as our present knowledge goes it is not likely that these animals can be assisted to emigrate in this way.

For the cocoons are rather bulky for this kind of porterage. Moreover they are apt to be deposited rather deep down and among the roots of grasses, and in situations where they are not so likely to become entangled in the feet of drinking birds. Assuming, however, that these difficulties can be got over there remains another difficulty. A single cocoon among the terrestrial Oligochaeta does not contain a large number of embryos, as has been pointed out on a previous page. It is true that Allolobophora foetida has six within one cocoon, but most of our indigenous forms have but from one to three embryos in a single cocoon. Thus, if successfully imported, it is hardly likely that the developed embryos scattered after their emergence would come together for breeding purposes; and in cocoons with but one embryo the accidental importation in this way would have to be very frequent to produce any result.

The case here is exactly the reverse of that afforded by the aquatic families (or many of them). In these Annelids the attachment of the cocoon to water plants, which are liable to be entangled in the feet of shore-frequenting birds, would tend to favour migration. And in addition to this the cocoons are naturally smaller and often contain a considerable number of embryos. We are to note that the aquatic forms are on the whole distinctly wider in their range than are the earthworms.

CHAPTER IX
THE GEOGRAPHICAL DISTRIBUTION OF
EARTHWORMS

The facts referred to and considered in the last chapter lead to further observations upon the geographical distribution of this group of animals and suggest problems for solution.