202 Prof. J. von Kennel on the 



supra-oesophageal ganglion is, in accordance with the invagi- 

 nation of the first segments, placed far back, while the 

 ganglion-chain is sometimes exceedingly concentrated, forming 

 a short band-shaped mass, from the lateral margins and 

 hinder end of which the nerves radiate into the segments after 

 the manner of a cauda equina — and sometimes also a well- 

 developed chain. If we retain the last case and assume a 

 great reduction in the number of body-segments, the con- 

 ditions in the Tardigrades present no further difficulty. 



Now, however, it is precisely among dipterous larvse that 

 we observe in a series of forms that they are capable of repro- 

 duction as larvae [Cecidomyia). In these larvee a head 

 is never developed : it remains after a fashion latent for 

 generations. The larval sexual organs, which are in this case 

 only female ones, are situated, in the shape of small paired 

 sacs, upon the dorsal side of the intestine and have no ducts ; 

 the differentiation of the germ-cells into &^g- and nutritive 

 cells appears to be abolished or at any rate not sharply 

 expressed. Now if we were to suppose that larvaB of this 

 kind had ceased to undergo their metamorphosis, that they 

 always reproduced their species as larvse, and that, in conse- 

 quence of adaptation to the very peculiar conditions of 

 existence in damp moss and water, they had become modified 

 in one direction and had then undergone further development, 

 we might regard them as constituting a transition to forms 

 which we now know as Tardigrades — Arthropods in the larval 

 stage, without a head, and with a body consisting of a reduced 

 number of segments, and bearing a few (secondary) leg-stumps. 

 In the course of this process a few peculiarities must naturally 

 have received especial emphasis. In the first place, not only 

 female but also male larvse must have remained in this stage. 

 Now we actually find, as, for instance, in the case of Cecido- 

 myia, that after a series of psedogenetic and parthenogenetic 

 larval generations the spontaneous production of male larvse 

 occurs, since finally both males and females appear as 

 imagines. The idea that under certain circumstances male 

 larvse of this kind also failed to undergo metamorphosis must 

 not be rejected offhand. We need simply and solely suppose 

 that the sexual organs gradually redeveloped a duct — in the 

 present case a short canal in communication with the rectum. 

 And this supposition is certainly not outside the limits of 

 what we otherwise concede to the capacity for modification 

 shown by the animal body. It is likewise conceivable that 

 independent efferent ducts, which were previously present, 

 came into communication with the rectum' through invagina- 

 tion of the posterior body-segments, while in this way the 



