THE HE A D CAPSULE. 113 



ganglia, and below the median commissure of the optic lobes 

 and corpora fungiformia. 



The nerves to the organs of the mouth and face arise from 

 the infra-oesophageal ganglia, whilst those which supply the 

 auditory organ are derived from the sixth or seventh neuro- 

 meres, or segmental ganglia, which in the insect are the 

 posterior thoracic ganglia. 



Biramous Appendages.^ — If, as Dr. Gaskell suggests, the limbs 

 of the arthropod correspond with the visceral arches of the 

 vertebrate, and the branchial clefts are the spaces between 

 them, we should have seven such clefts between the auditory 

 capsule and the mouth, representing the cephalo-thoracic limbs, 

 corresponding with seven pairs of cephalic metameral nerves 

 as the primitive number in vertebrates. That no vertebrate is 

 known in which these arches exist does not, however, militate 

 against the argument, as the persistence of the cranial nerves 

 renders it certain that a number of somites have disappeared 

 in the process of evolution. 



The double character of the embryonic appendages in the 

 Crustacea, and in the maxillae of insects, as well as in the 

 thoracic limbs of the rudimentary fly nymph, is certainly very 

 suggestive of the double character of the pterygomaxillary arch, 

 or even of the hyomandibular in vertebrates. 



It is true Balfour [43, vol. i., p. 542) states that in insects 

 double or biramous appendages never exist ; but this statement 

 is certainl}^ erroneous, though, except in the nymph of the 

 Diptera, the thoracic legs have not hitherto been seen in insects 

 in the biramous stage. I cannot doubt that the biramous 

 thoracic feet of the fly nymph, described by Weismann, and 

 seen by myself, are indicative of a primitive biramous con- 

 dition. 



Development and Morphology. — The head of an insect consists 

 undoubtedly of a pre-oral and a post-oral portion, and the 

 latter is formed from three segments (PI. V., Figs, i, 2, and 3), 

 whilst the former, although divided into three distinct regions, 

 is only segmented in a very different sense ; it does not exhibit 

 a series of metameres. 



