200 Prof. J. von Kennel on the 



of the antennas as due to reduction, which may well be the 

 only possible explanation, it would follow that the first three 

 pairs of appendages of tlie Tardigrada are homologous with 

 the mouth- parts of the Tracheata, and only a single segment 

 would be left for the body. This would entail the conclusion 

 that the rest of the trunk-segments of the Tracheates have 

 arisen in consequence of continued segmentation on the part 

 of the common ancestors of Tardigrada and Tracheata. 

 Although this would be an absolutely permissible suppo- 

 sition, we must nevertheless consider that in the Peripatiis- 

 like Protracheata we have animals which, with a larger number 

 of segments, combine Annelidan characters even in a far 

 greater degree, and consequently fulfil all demands that can 

 be made upon transitional forms. Moreover all Myriapods, 

 from which the rest of the Tracheata can be derived by the 

 process of concentration of segments &c., possess altogether a 

 larger number of segments and a fully developed head, the 

 rudiments of which are already in process of formation in 

 Peripatus in the shape of antennae, jaws, and slime papilla. 



Let us now consider whether the peculiar conditions of the 

 Tardigrada cannot after all be derived more easily and simply 

 from higher Tracheates by the method of reduction and 

 simplification. Let us just take the Tardigrade body as we 

 see it — a segmented animal which, according to its nervous 

 system, consists of five segments. It must certainly be 

 admitted that we are acquainted with no Tracheate Arthropod 

 which is composed of so few segments. But we nevertheless 

 find in the entire Tracheate stem a reduction of the number of 

 segments from the Myriapods upwards, which is brought 

 about in part by the actual absence of posterior abdominal 

 segments, and in part by fusions and intimate union of 

 several segments. As a general laile among the higher 

 Tracheata four cephalic, three thoracic, and a somewhat 

 fluctuating number of abdominal segments are present ; the 

 latter, however, in the Mites and the true Spiders are fused 

 into one piece, which is greatly reduced in the case of the 

 former. A reduction such as this may have occurred in other 

 Tracheata also. Moreover, we are acquainted with stages of 

 Tracheates, namely their larvas, in which the divisions of head, 

 thorax, and abdomen are not differentiated ; larvae in which 

 the appendages have disappeared in adaptation to certain con- 

 ditions of existence (maggots) ; and, finally, larvse in which 

 new stumps of appendages, in the form of simple dermal 

 protuberances provided with claws of different shapes, have 

 appeared secondarily upon all the segments of the body or 

 upon a portion of them. I merely recall the caterpillars with 



