256 EDWIN S. GOODRICH, 



by Laukester iu 1881, that the ganglia of primitively post-oral 

 raetaraeres have shifted forwards, to fuse with the primitive 

 Annelidan brain, the arcliicerebrum, to form a syncerebrum of 

 compound structure, has been amply supported by the facts 

 of comparative anatomy and embryology.^ But this suggestion 

 must not be pushed too far ; not every lobe, not every epidermic 

 invagination or centre of proliferation in the embryo must be 

 taken for a metameric ganglionic mass. We know, as already 

 mentioned, that the uusegmented archicerebrum of Annelids 

 may be much lobed and differentiated, and may even arise 

 from several separate centres in the prostomium itself. An ap- 

 parent '' neuromere " can only be accepted as of metameric value 

 when the interpretation is supported by evidence derived from 

 other parts, such as the mesoblastic somites and the appendages. 

 Let us now examine the various groups of Arthropods. 



The Peripatoida (figs. 5 and 6). 



In Peripatus, the most worm-like Arthropod, we find a head 

 bearing a pair of antennae and eyes, and two pairs of more 

 posterior appendages modified in relation to the mouth — the 

 mandibles and oral papillae. 



A study of the development has sliown that the head of 

 Peripatus is formed of three segments. All observers are 

 agreed that the posterior two, to which the oral papillae and 

 mandibles belong, are genuine metameres ; but some doubt 

 exists as to the nature of the preoral segment bearing the eyes 

 and antennae, many writers having compared the antennae to 

 the prostomial tentacles of Annelids. Von Kennel and Sedg- 



* "In the Chsctopoda, tlie pre-cesopliageal gauglion appears always to 

 remain a pure archicerebrum. But in Crustacea (and possibly also all 

 other Arthropoda, though there is a case to be considered for Peripatus and 

 for the Hexapoda and Myriapoda, on the supposition that their antennae are 

 not the equivalents of Crustacean antennae, but of the processes of the cephalic 

 lobe of Clisetopoda) the pre-oesophageal ganglion is a syncerebrum, con- 

 sisting of the archicerebrum and of the ganglion masses appropriate to the 

 flrst and second pair of appendages, which were originally post-oral, but have 

 assumed a pieoral position whilst carrying their ganglion masses up to the 

 archicerebrnm to fuse with it." — E. Ray Lankester (10). 



