Palaeozoic Arachiiida of North America. 15 



phylogenetic value and does it justify the expansion of the class to 

 include also the Trilobites and Pantopods ? 



Numerous embryological studies have given an affirmative an- 

 swer to the first question. It has been shown by Heymons that in the 

 Chilognatha, of the three pairs of post-oral embryonic segments desig- 

 nated respectively as prae-antennal, antennal and prae-mandibular, 

 the second develops the antennae of the adult, the first forms transient 

 appendages, while the third disappears altogether at an early stage. 

 The fourth post-oral segment gives rise to the mandibles. In insects 

 we have a somewhat similar condition, the antennae arising from the 

 second post-oral segment while the third segment gives origin to a 

 temporary, embryonic pair of appendages. The protencephalon, on 

 the other hand, representing the first, prae-oral segment, is divided 

 into three lobes, the first of which develops into the optic ganglion 

 while the second and third give rise to the brain proper. In the adult 

 insect the procerebral lobes of the brain give rise to the nerves of the 

 ocelli or simple eyes. The class Crustacea is somewhat different 

 from the two preceding ones. Here it is more or less customary to 

 regard the eyes as modified appendages of the first prae-oral segment 

 and this view has gained some support in the experiments of Herbst, 

 which show that in place of an amputated eye under circumstances 

 an antenna may develop. But it is far from being e^^dent that 

 regeneration repeats past history. Double members ma5'' be produced 

 artificially in lower vertebrates by splitting the germ of the antei ioi 

 or posterior leg and it is clear that they do not represent structures 

 which once existed but have been lost. Moreover, the early larva of 

 the lowest crustaceans, the so called Nauplius, has only three pair? 

 of appendages, all of which belong to post-oral somites. Of these 

 appendages the first pair becomes the antennules, the second the 

 antennae and the third remains post-oral and changes into the man- 

 dibles. The eyes of higher crustaceans develop later and occupy 

 a position in front of the antennules. 



Among the recent Arachnida the spiders have been studied by 

 different investigatois and their embryology is better known than 

 that of any other order. It is beyond any doubt that the cheliceral 

 segment appears originally as a post-oral metamerite and occupies 

 later the position in front of the mouth. There is, however, a great 

 divergence of opinion as to whether the eyes represent a separate 

 segment and whether they are homologues to the eyes of crustaceans 

 and insects. In a recent paper on the development of Ischnocolus, 

 Schimkewitsch Writes : 



