NEW SPECIES OF perichj:ta. 223 



guide to the identification of a worm to know about what size other 

 presumed similar forms may be. The spermatheca is another 

 organ that is of general diagnostic value. Michaelsen remarks, 

 no doubt with truth, that it contracts in spirit. So does the 

 whole body and every organ of the worm ; but, nevertheless, the 

 general shape and proportions of sac to diverticulum would not 

 greatly alter. Of much greater importance is the fact that the 

 size depends on the presence of spermatozoa; for we know that 

 in Lumhricus hermdeus the worm may have fully-developed 

 sperm-sacs and clitelium, but the spermathecse are frequently 

 extremely minute, even in the freshly-killed worm, Yery 

 probably they would be invisible if that worm had been preserved 

 in alcohol; and it is quite within the bounds of probability 

 that P. atlieca^ P. acystis, and others, which have been described 

 as being without spermathecse, are in the condition of the 

 Lumbricus just referred to, i. e. that the spermathecse are not yet 

 functional. 



Michaelsen also objects to the value of the position of the 

 spermathecse, whether they lie, for example, in the 7th or the 

 8th segment, and open to the exterior between these segments. 

 Here, again, variability may occur : a worm with its spermatheca 

 normally in segment 8 may sometimes have it in 7. Several 

 authors have described such variations. Nevertheless, out of the 

 hundreds, or more probably thousands, of specimens of L. liercu- 

 leus that have been opened under my supervision, and have been 

 drawn by students, so that I have been able to note any departures 

 from the normal, I have only noted such variation in the position 

 of the spermathecae some two or three times. Until we know more 

 of the variability of the animals we are justified in regarding 

 a given position as fixed, if any considerable number of specimens 

 reveal it ; and it appears to me that all lumbricologists describing 

 new species should state explicitly the number of specimens they 

 have examined, and should be in a position to state how far 

 such and such an organ is subject to any alteration in size and 

 position. 



Too frequently no mention is made of the number of speci- 

 mens upon which a species is founded; still more frequently a 

 new species is founded for one single specimen. I have done this 

 myself; and it serves to call attention to some new permutation 

 of characters which, later on, may be proved to be mere 

 variations from some well-known "type" species. Michaelsen, 



