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tion is probably incorrect in this instance, as the nematoid worm 

 above mentioned has been found by Eschricht to possess sexual or- 

 gans, the Filaria medinensis, in my opinion, does present an instance 

 of a transition form of the nature alluded to. It must be confessed, 

 however, that as yet the existence of an anterior and succeeding form 

 to that with which we are acquainted in the Guinea worm, rests only 

 upon circumstantial evidence. The intermediate form in all those 

 animals in which the mode of development by means of alternate ge- 

 nerations obtains, or as it is termed by Steenstrup, the "nursing" 

 generation, though of various kinds in other respects, presents in all 

 cases certain common distinctive characters. 



1. It is always female in its attributes. 



2. The modification which the female organism undergoes for the 

 purpose of becoming a "nurse," at all events, when destined for that 

 class of "nurses" in which the third or fourth generation, as the 

 case may be, is to be developed, consists in the abortion of parts of 

 the generative tube, viz., the ovaries and oviducts. "Nurses" of this 

 kind may, in fact, as Steenstrup says, be considered as individualized 

 " uteri." 



3. It is always proligerous, never oviparous. 



Now if we consider the anatomical structure and the external rela- 

 tions of the Guinea worm with reference to the above particulars, we 

 shall find that it presents all the characters of a " nursing " or inter- 

 mediate generation. 



1. It is unisexual and female. 



2. The female organs are modified, so that no trace of an ovary or 

 of anything but what may be considered a uterine cavity remains. 



3. It is also necessarily only proligerous. 



The truth of the first proposition of course depends upon the cer- 

 tainty of there being no male Filaria medinensis, as I have before 

 observed. The second and third propositions are sufficiently ob- 

 vious, and in fact are so obvious that M. Jacobson compares the 

 body of the worm to a tube or sheath inhabited by the young ones, 

 than which, a more accurate definition of one of Steenstrup's 

 "nurses" could scarcely be given. 



We are, however, still deficient in any knowledge of some very 

 important points connected with this worm, and which it is neces- 

 sary should be determined before the theoretical view of its nature I 

 have ventured to propose can be deemed anything more than an un- 

 certain speculation. 



But a strong proof in favour of the opinion that the Guinea worm 



