428 SCIENCE PROGRESS 



which thus corresponds to a double segment (two fused seg- 

 ments). Probably the Diplopoda have been derived from this 

 order by a process of reduction but the earliest known type 

 of this order— J itlopsis cretacea — occurs only in the Cretaceous 

 of Greenland. 



There are several points in which the Archipolypoda 

 can be held to approach the Trilobites, from which some 

 zoologists consider them to be derived : the fusiform body is 

 thickest in the anterior half or third; the cephalic appendages 

 are borne on an apparently single segment. Stigmata occur on 

 each segment, so that they were presumably air-breathers ; but 

 Scudder^ considers that the lateral openings of Acantherpestes 

 were branched in character, so that this type would help to 

 bridge over the transition which must have occurred in the 

 Devonian period between gills or gill-books and tracheae. 



In the Archipolypoda the pleurae are. well developed, and 

 in the modern Polydesmus (fig. 19), to which the Devonian 

 Archidcsmidce show much resemblance, we can even yet trace 

 a distinct trilobation of the body comparable with that of Trilo- 

 bites. The single pair of antennae also is a characteristic which 

 the Myriapods possess in common with Insects and Trilo^ 

 bites, in abrupt contrast to all Crustaceans (with two pairs) or 

 to Arachnids (with only chelicer£e). In the larva of Polydesmus 

 the lateral cheeks of the head present a striking analogy to the 

 free cheeks of Trilobites ; here too there are only three pairs of 

 legs on the three anterior trunk-segments, presenting an interest- 

 ing parallel to the permanent reduction to three pairs in insects. 

 New segments gradually appear posteriorly and the number of 

 legs increases, a state of things very analogous to the increase of 

 vertebral segments in snakes or to the addition of the fifth pair 

 of legs in some Pycnogons (Decolopoda and Pentanymphon), 

 where it is a comparatively new development and not a primitive 

 character ; in fact the larval P3Tnogon has only three pairs of 

 appendages. 



As a set-off to this increased number of similar segments, 

 which is obviously a secondarily acquired characteristic (al- 

 though it must have occurred early in the history of the group 

 of Myriapods), the Symphyla show a marked reduction, possess- 

 ing not more than twelve leg-bearing trunk-segments, with 

 but one pair of legs to each segment and only a single pair of 

 ' Mem. Bost. Soc. Naf. Hist. 1882 and 1884. 



