Affinities and Origin of the Tardigrada. 203 



reduction in tlie number of the segments would also become 

 more intelligible. The enigmatical " gland," which occurs 

 in both sexes of Tardigrades [of. Plate, loc. cit.), is perhaps 

 nothing else than the degenerate second ovary or testis, just 

 as in the case of birds also, at least in the female sex, only 

 one half of the genital apparatus arrives at maturity. 



Now it is by no means my intention to put forward dipterous 

 larvEe as actually the ancestors of the Tardigrada; on the 

 contrary, I merely selected these larvee in particular because 

 they combine in themselves a series of peculiarities which 

 show how great the capacity for modification may be in such 

 animals, and because these peculiarities occur in a precisely 

 similar manner in the Tardigrada. Just as in the case of 

 Diptera such very far-reaching secondary changes were possible 

 in the larval stage in adaptation to certain conditions of 

 existence, so this might equally well have happened in the 

 case of the larvae of other insects also of which we have lost 

 all knowledge. I merely mean that, of all tracheate Arthro- 

 pods with which we are at present acquainted, no single form 

 so simply and so readily enables us to interpret the Tardi- 

 grade body as these very dipterous larvae. 1 do not believe 

 that the Tardigrades can be placed at the root of the tracheate 

 stem or in the neighbourhood of it ; for the conditions of their 

 organization diverge more from those of the Annelids than 

 do those of indisputable Tracheates of much higher rank. If 

 my memory serves me it was once declared by Hay Lankester 

 that in the case of animals of very small size but of relatively 

 complicated structure we must first direct our thoughts towards 

 degeneration and reduction from higher forms. If we derive 

 the Tardigrada in the manner indicated above from paedo- 

 genetic and greatly modified Tracheate larvee we can regard 

 the entire body of these animals as an Arthropod trunk of 

 four segments, of which the head ceased to be developed, and 

 of the cephalic organs of which the supra-oesophageal ganglion 

 is the sole remnant. The first three ganglia of the ventral 

 cord, which in higher Arthropods are fused together to form 

 the suboesophageal ganglion and innervate the mouth-parts, 

 may very well in the course of time have degenerated and 

 disappeared, since the organs which they had to supply were 

 no longer developed. The four ganglionic centres of the 

 Tardigrades may then correspond to three thoracic and one 

 abdominal ganglion. If we consider that the young of many 

 Myriapods leave the egg at a very early stage with quite a 

 small number of segments, and only develop the remainder 

 during free existence, we can also conceive that this may have 

 been possible several times, and that such immature larvae 



