I902. Friknd. — Studies in Irish E^ichytrcsids, 113 



Enchytrcetcs'^. My own suspicion is that the smaller and 

 earlier form may be regarded as in some sense the larval 

 stage of the later and larger. 



I^et us examine the possible explanations. If the problem 

 had been submitted to me from without, in such a way that I 

 had not had the case immediately under my own personal 

 observation, I should have said : — 



1. We may have here a case of symbiosis, or a kind of free 

 parasitism. One is so familiar with this kind of thing in 

 nature, that even if symbiosis among annelids is not yet proved, 

 there is no reasonable ground for supposing it improbable, 

 We know so little about the life-history of these lowly creatures 

 that we cannot at present say what novelties may not be dis- 

 covered among them. Perhaps, then, we have a case of 

 symbiosis before us. If that may not be, then — 



2. The first and smaller form may be regarded as a pioneer, 

 preparing the way for the larger and later form. This, again, 

 is a frequent occurrence. We know that there are many 

 creatures which cannot win a livelihood except in the tracks 

 of some other worker. It may be the two are related, or they 

 may be widely separated forms. Some pCvSts never initiate 

 sickness or decay, but they speedily rush in when weakness 

 has shown itself. So possibly the Aster W^orm strikes the blow, 

 and the larger species follows it up. That is a second 

 hypothesis. 



3. It is, however, not in the least contrary to fact or reason 

 that the second and larger worm is a higher or adult form of 

 the Aster Worm. The following are some of the arguments in 

 favour of this theory. In the first place my experience 

 hitherto shows that while the Aster Worm {E. parvulus) is the 

 form most frequently found when disease begins to show 

 itself, as time goes on the second form increases in numbers 

 till they are equal — as in the case of the worms treated of in 

 the foregoing section. Finally, the lesser form disappears 

 altogether, and the larger takes the field, as in my own celery 

 trench in 1898. I do not think there is any evidence at present 

 to show that the larger and later form devours, ousts, or in any 

 other way drives its lesser rival from the field. 



Next I may add that, so far as I have been able to observe, 

 it is somewhat rare for the lesser form {parvulus) to show 



A 3 



