Affinities and Origin of the Tardigrada. 199 



lature of tlie Bear-animalcdes, too, diverges from tlie dermal 

 muscle-sheath of the Annelids much further than does the 

 musculature of Peripatus ; they have isolated muscle-strands, 

 which run freely through the body-cavity, decussate, and are 

 inserted at different points of the integument ; in accordance 

 with their small size these are of very simple structure and 

 consist of few fibreSc This arrangement reminds us forcibly 

 of the distribution of the muscles in many highly developed 

 Tracheates, which will be discussed later on. The nervous 

 system of the Tardigrada can, it is true, be easily compared 

 with that of the Annelids, but it also differs in no way from 

 that of higher Tracheates, in which the sense-organs of the 

 head are slightly or not at all developed. At any rate in its 

 configuration as a chain of ganglia it surpasses that of Peri- 

 patusj which has a very irregular rope-ladder-like nervous 

 system. On the other hand, however, the eyes of the Tardi- 

 grada are such insignificant dots of pigment that we can 

 scarcely find more simple ones among the Annelids ; they can 

 only be regarded as rudimentary structures, but not as engaged 

 in progressive development. 



While the sexual organs of Peripatus can at once and by 

 means of embryological proofs be traced back to those of the 

 Annelids, and their ducts to segmental organs, this is not 

 possible in the case of those of the Tardigrada, since in the 

 first place they are unpaired, and, secondly, open dorsally into 

 the posterior portion of the intestine. This condition also is 

 more readily intelligible on the theory of reduction and dege- 

 neration, as I shall subsequently show. 



If we regard the Tardigrada as an offshoot from the root of 

 the Tracheata, we certainly need make no attempt to think 

 out the conditions of their organization in their further deve- 

 lopment into Arthropods. If, however, they " most clearly 

 express the transition from the Annelids to the air-breathing 

 Arthropods," the ancestors which were common to them and 

 the Tracheata would yet have to be supposed to have 

 resembled them fairly closely. But here great difficulties 

 crop up in relation to the appendages. The four pairs of 

 appendages of the Tardigrada are post-oral, and are inner- 

 vated from the ventral chain of ganglia, while the Tracheata 

 nevertheless possess a pre-oral pair of appendages in the shape 

 of the antennae. 



Now this pair of appendages must either have disappeared 

 in the Tardigrada or it must be regarded as a new develop- 

 ment in the rest of the Tracheata ; for the first pair of leg- 

 stumps of the Bear-animalcules is innervated from the first 

 ventral ganglion. Should we wish to interpret the absence 



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