286 IN VERTEBEATA CHAP. 



appendages, primitive in most other respects, and preserved up to 

 the present day in consequence of its secluded and burrowing habits. 

 The Diplopoda and other Progoneates would represent another 

 degenerate offshoot, whilst the Chilopoda and Insecta Hexapoda 

 would constitute the main stem. 



The land Arachnida may plausibly be regarded as later invaders 

 of the land. They, too, are connected with the Trilobite stem, but 

 more remotely, for in them a differentiation between prehensile and 

 ambulatory legs in front and blade -like respiratory appendages 

 behind, had been effected before the ancestral Arachnid left the 

 water. Crustacea can also 'be directly traced back to primitive 

 Trilobita; indeed the Nauplius larva, as we have already seen, is 

 in one sense the repetition of the Trilobite stage in Crustacean 

 ancestry. 



The main difficulty connected with this hypothesis concerns the 

 origin of the compound eye. Since Peripatus, Myriapoda, the lower 

 Insects, and the larvae of the higher Insects, agree in possessing only 

 simple pit-like ocelli, it is fairly clear that the massing together of 

 these ocelli to form a compound eye must have occurred during 

 the evolution of the Arthropodan stock, and had not occurred in 

 the primitive land Arthropoda. But primitive Crustacea, primitive 

 Arachnida, and most Trilobita possess compound eyes. The Insecta, 

 therefore, must have been derived from primitive Trilobita in which a 

 compound eye had not yet been developed, and, considering that a fully 

 formed Orthopteran insect is already found in Silurian strata, this is 

 what we might naturally expect. 



One suggestion may be made here of a possible future line of 

 research, though it must be understood it is merely an indication 

 of such. In Trilobita, primitive Arachnida, and very many Crust- 

 acea, the compound eye is carried on a lateral projection of the 

 prae-oral region, and in the higher Decapod Crustacea this be- 

 comes a movable appendage. In shrimps, when this appendage 

 is 'cut out along with the optic ganglion it is regenerated, not as 

 an eye, however, but as an antenna-like organ. The question is 

 Is it possible that in the compound eye of Trilobita, Crustacea, 

 and Arachnida we have the representative of the prae-antennae 

 of Scolopendra, and that the compound eye of Insects has had 

 an independent origin ? This is a subject which is well worth 

 investigation. 



Before leaving the subject of the Insecta, however, another very 

 difficult problem, and one of fundamental importance for embryo- 

 logical science, presents itself, and that is the question What 

 significance are we to attach to the larval phases of Insects ? We are 

 confronted with the extraordinary circumstance that the lowest insects, 

 like Machilis, Lepisma, etc., are hatched from the egg in practically 

 the adult condition ; that the more primitive groups of winged 

 Insects, like Orthoptera, Odouata, etc., emerge from the egg in a 

 condition fundamentally like the adult, except for the absence of 



