^^8-;;:- ARE THE ARTHROPODA A NATURAL GROUP? 265 



passage of metastomial parapodia into association with the prostomium 

 (and therefore a praeoral position) is what with all reserve and caution 

 they hold to afford the best explanation of the facts at present known. 

 It appears that the antennae of Peripatiis and the antennae of myrio- 

 pods and hexapods as well as the two pairs present in Crustacea are 

 all of them post-oral parapodia shifted in relation to the mouth. No 

 adult arthropod retains the prostomial antennae of the chaetopod, but 

 Korschelt and Heider suggest that the strange pair of small processes 

 seen in embryo Peripatus in front of the head are these prostomial 

 antennae in a transient condition, whilst they are also represented (as 

 I have myself long since taught) by the frontal tentacles of certain 

 crustacean Nauplii (Cirripedes). The prostomial eyes of the chaetopod 

 appear (?) to be retained hy Peripatus, but the eyes of all other Arthropoda 

 are a new structure, and it is not impossible that they may after all 

 represent (cf. Herbst's experiments on regeneration of crustacean eye 

 as an antenniform palp) a pair of fore-shifted parapodial appendages. 



The conclusion to which embryological study (as to segments of 

 body-cavity, so-called cerebral ganglion, and position of early rudi- 

 ments of antennary appendages of arthropods) tends, is that in the 

 various branches of arthropod descent from the ancestral Pro- Arth- 

 ropoda there have been forward movements or shiftings of post-oral 

 segments and their parapodia (possibly to the number of three in 

 some cases and of less in other cases), so that the mandible oi Peripatus 

 does not belong to the same ancestral segment as does the mandible 

 of the insect and crustacean, whilst the cephalic appendages of the 

 Arachnida require a separate interpretation, {cf. Korschelt and 

 Heider, " Vergleich. Entwickelungsgesch." p. go6). The development 

 of my original doctrine of prostomiad shifting or attraction of post-oral 

 segments and their appendages to the prostomium has indeed now 

 assumed very serious proportions. It will require renewed researches 

 both upon the developmental history of the chaetopod prostomium 

 and that of the head in various Arthropoda, before precise and definite 

 conclusions in the matter can be formulated. 



In the meantime I am none the less unable to agree with those 

 who doubt that the Arthropoda form a natural group in the sense 

 indicated above. 



There are two remarkable and independent characters possessed 

 by all Arthropoda, including Peripatus, which we must suppose either 

 to have been independently acquired by the different lines of descent 

 now united under the name Arthropoda — or to have been inherited 

 by all Arthropoda from common ancestors — the Pro-arthropoda. 

 These two characters are (i) the modification of one or more pairs of 

 post-oral parapodia to serve as opposable nipper-like jaws (hence the 

 term Gnathopoda which I formerly suggested as more appropriate 

 than Arthropoda for the designation of the group) ; (2) the peculiar 

 condition of the great dorsal heart, which may be termed ' ostiate.' 

 The Arthropod heart is a contractile tube, lying in a pericardial 



