148 OLIGOCHAETA 



tnvlti'porus never more than one. In Allolobophora putra, cMorotica, and in some 

 others, three is the number; Megascolex coeruleus has two embryos— at least this 

 was so in two cocoons of this species which I examined. In Allolobophora foetida 

 there are two to six embryos. In many of these cases there are probably more ova 

 in the cocoon than the few which give rise to worms ; this is certainly the case 

 with Allolobophora foetida, in which species Vejdovsky states that there are about 

 twenty ova; those which do not produce mature embryos do not do so for the 

 following reasons: either the ova are not capable of fertilization; or they are not 

 fertilized ; the egg division takes an abnormal course in an unfavourable direction ; 

 or the formation of ' twins ' ^ takes place. Similar variations in the number of 

 embryos within a single cocoon occur in the family Enchytraeidae ; in some species 

 there is but a single embryo ; in others as many as sixteen. 



The cocoons of the Oligochaeta are deposited in very various localities; Rhyn- 

 chelmis attaches them to aquatic plants; those of Ilyodrilus are found in groups 

 fixed to the roots of aquatic plants ; many earthworms deposit theirs superficially 

 upon the soil; others, on the contrary, lay them deep down in the ground. Those 

 of Megascolex coeruleus are laid in a deep burrow ; so also are those of Allolobophora 

 trapezoides, according to Duges (6 inches deep). Octochaetus muUiporus selects a 

 curious locality ; the cocoons of this species, described by me some time since, were 

 found at the edge of a swamp, which in wet seasons is covered with several feet 

 of water. 



X. Geographical Distribution 2. 



In considering the distribution of the group, ^nd in drawing conclusions therefrom, 

 it is necessary to be very cautious in laying too much stress upon positive arguments. 

 Negative arguments, i.e. the non-occurrence of a species in a given locality, are 

 from some points of view more important. The reasons for this perhaps somewhat 

 paradoxical conclusion are chiefly the accidental importation of worms from one 

 country into another by man's agency. There is absolutely no doubt that this 

 takes place ; I have myself received earthworms from various countries accidentally 

 included in Wardian cases : again, various species of Perichaeta have been met with 

 in this country and in America in hot-houses only. On the other hand, it is perhaps 

 rash to go to the opposite extreme, and to explain every inconvenient fact in distri- 

 bution by assuming accidental importation. The facts must be treated with discretion. 



1 It is not unusual, as has been described by Kleinenbers and Vejdovsky (0), for there to be double 

 embryos, of which one only arrives at maturity. 



' For the details of distribution see the systematic part of this volume. 



