and the Genealogy of the Arthroj^oda. 453 



which is destitute of appendages, is composed of ten segments. 

 By the fusion of the head and three thoracic segments we get 

 the primary form of the Arthrogastres (Scorpions, &c.), and 

 by the further fusion of the abdominal segments into one piece 

 the Sphoii-ogastres (true Spiders). 



The Myriopoda have broken out from some early insectan 

 branch. This is clearly shown by their embryonal form and 

 development ; for the embryos possess only three pairs of legs, 

 and perfectly resemble larvfe of insects. Moreover the in- 

 ternal anatomy of the Myriopoda is so nearly related to that 

 of insects that there can be no doubt of the fact of their 

 derivation. The great number of body-segments, and con- 

 sequently of legs, is a subsequent addition, acquired after the 

 branching off, as is proved by their development, and is also 

 shown by the analogy of many Crustacea (Edriophthalma). 



Thus we come to the true Insecta. Here, following the 

 example of Fritz Miiller and Hackel, we must in the first place 

 dispose of a strong prejudice, namely, the principle of classifi- 

 cation according to the '^complete" or "incomplete" meta- 

 morphosis. This is now-a-days a perfectly untenable prin- 

 ciple. We now know not only what is the significance of 

 metamorphosis in general and what we are to conclude there- 

 from, but we have also learnt, thanks to the brilliant investi- 

 gations of Fritz Miiller upon the Crustacea, what modalities 

 may affect the metamorphosis, lengthening, abridging, or 

 altering it ; and we know that the so-called '^ perfect " meta- 

 morphosis of many, and perhaps of all insects, has been 

 acquired during ontogenesis (and not inherited from the ori- 

 ginal progenitor). Moreover we have obtained from the facts 

 the abstraction that the metamorphosis is always abridged in 

 proportion as more generations follow one another, and that 

 the tendency of the organisms (if we may use the expression) 

 is always striving to attain, by the shortest possible way, from 

 the Q^g state to the perfect, sexually mature animal. For this 

 reason I have already indicated how important, and how rich 

 in unexpected results, a comparative investigation of larvae 

 will be. One of the most striking examples of a perfect dif- 

 ference of metamorphosis, with the greatest similarity in its 

 starting and finishing points (the tgg and the sexually mature 

 animal), is presented by a species of the genus Gecarcinus, a 

 Brachyurous Crustacean which, like the Crayfish, quits the 

 Qg^ at once in its definitive form, whilst all other crabs, and 

 even all other species of the genus Gecarcinus^ only attain 

 their definitive form after passing through a metamorphosis. 

 Similar peculiarities will certainly be presented to us by a 

 careful investigation of larvse, and the notions of complete and 



