vm ARTHROPODA 285 



according to Heathcote (1888), originates by the fusion of the 

 ventral portions of coelomic sacs, and is in fact a remnant of the 

 coelom. The mid-gut epithelium is formed by an accumulation of 

 yolk cells, not, as in Scolopendra, on the surface of the yolk, but in the 

 form of a string traversing its centre. It follows that as development 

 proceeds the yolk is situated, not in the cavity of the gut, but in the 

 body-cavity, so that between Diplopoda and Chilopoda there is the 

 same difference as between Gnathobdellidae and Rhyncobdellidae 

 among Leeches. 



The embryo is hatched as a larva, when, in addition to jaws, it has 

 only three pairs of legs. Additional legs are added in subsequent 

 moults. Scolopendra, on the contrary, is hatched with all its 

 appendages developed, and this is true of many Centipedes which are 

 termed on this account Epimorpha. But there are many Centipedes 

 which agree with the Millipedes or Diplopoda in being hatched with 

 an incomplete number of segments. Such forms are termed 

 Anamorpha. 



GENERAL CONSIDERATIONS ON THE ANCESTRAL HISTORY OF ARTHROPODA 



We may now review the whole of the facts which we have learnt in 

 order to see what light they throw on the relationships of the Insecta, 

 Myriapoda, and Hexapoda, and the Onychophora. Since in their 

 embryonic development most insects pass through a stage where 

 appendages are developed on the segments of the abdomen, and since 

 in the lowest Insects such appendages persist in a rudimentary form 

 throughout life, there is no doubt that Insecta Hexapoda are derived 

 from Myriapoda ; and since, like Scolopendra and Peripatus, they 

 have the genital opening situated at the hinder end of the body, they 

 must be derived from some opisthogoneate stock bike the Chilopoda, 

 rather than from progoneate forms. 



To find the origin of these last named we must go back to an 

 ancestral condition where there were many genital openings on each 

 side of the animal, a condition retained only in some Pantopoda, and 

 one in which the ancestors of Myriapoda cannot have made much 

 advance beyond that found in Polychaeta. 



It is a tempting hypothesis, however, to suppose that such a 

 state may have existed in the extinct Trilobita. In this group 

 there was more uniformity in the character of the appendages borne 

 by the various segments than in any other long-bodied Arthopod, and 

 moreover they agreed with Insecta, Myriapoda, and Onychophora in 

 possessing a single pair of antennae as prae-oral appendages. 



If this hypothesis be accepted we should be dealing, in the case 

 of Insecta, in the broad sense, with a land branch of a stock which 

 we know to have been dominant in the seas of the Cambrian and 

 Silurian epochs. The Onychophora would then be a somewhat 

 degenerate offshoot from the base of this stem, degenerate in the 

 thinness of its cuticle and in the loss of distinct joints in its 



