Affinities and Origin of the Tardigrada. 201 



their " pro-legs " and many dipterous larvse with leg-stumps. 

 Most important of all is the fact that in dipterous larvaa of this 

 kind the entire head is wanting, since the four foremost seg- 

 ments of the body are invaginated and constitute the oesophagus 

 of the larva. 



In the metamorphosis of the larva all portions of the head 

 of the imago are formed in a reversed position out of rudi- 

 ments in the wall of the larval oesophagus, and are everted in 

 the pupa. That which is known to us as a maggot is merely 

 the trunk, externally wholly unsegmented, enveloped in a 

 resisting cuticle, and apodous or with a very variable number 

 of secondary leg-stumps. If we imagine the trunk of such a 

 dipterous larva reduced to four segments we have in essentials 

 precisely the body of a Tardigrade. 



In the secondary oesophageal tube formed by invagination 

 of the four cephalic segments there lie in the case of the 

 maggots of Diptera the two stylets of chitin as masticatory 

 organs of local origin, not homologous with any appendages, 

 precisely like the chitinous rods in the oesophagus of the 

 Tardigrada. The musculature of a dipterous maggot is 

 certainly more complicated than that of a Tardigrade, but 

 exhibits so surprising a similarity in arrangement and distri- 

 bution that we are involuntarily impelled to institute a 

 comparison. It is true that the muscles of the Tardigrada 

 are not transversely striated, but this, in the light of present 

 views, will surely not carry much weight. In the larvae of 

 many Diptera {Stratiomys, Mycetophilidee, &c.) leg-stumps 

 occur in different numbers as independent new formations. I 

 myself when a student, some seventeen years ago, found 

 beneath a stone in the Black Forest a number of very peculiar 

 dipterous larvse, which I have never yet been able to deter- 

 mine and which bore upon the flat ventral surface four pairs 

 of such leg-stumps provided with claws. Salivary glands 

 and Malpighian vessels are present in the larvai of Diptera. 

 How greatly the condition of the alimentary canal can alter 

 according to the mode of life is shown with the greatest 

 distinctness precisely by Insect larvge. Thus we next come to 

 the trachea3. These are, indeed, present in larvee of Diptera, 

 but the number of the stigmata is considerably reduced; that 

 tracheie, however, may disappear, especially in very small 

 Arthropods which live in damp surroundings, is shown among 

 others by the Acarina. 



We have now only to deal with the nervous system and 

 sexual organs. 



The nervous system varies exceedingly in dipterous larvae, 

 even in those which we may designate as maggots. The 



