i66 
The Irish Naturalist. 
Oct., 
region is so greatly reduced that the legs seem attached 
close together in the mid- ventral hne, this modification 
involving a corresponding displacement of the spiracles. 
The air-tubes of millipedes are far simpler than those of 
centipedes. And it is well known that in the great majority 
of milHpedes most of the primitive body-segments become 
fused together in couples, so that each apparent segment 
in the adult carries two pairs of legs. No doubt can be 
entertained — if these considerations be given due weight 
— that centipedes are more nearly related to insects than 
they are to millipedes. If millipedes and centipedes be 
placed in one class, insects must also be included in the 
same class, which can then no longer be termed " Myria- 
poda." But the more reasonable arrangement by far is 
to treat each group as an independent class. 
Another important distinction between centipedes and 
millipedes is found in the appendages of the head. In a 
centipede, such as Lithobius, there are elongate feelers, 
mandibles, two pairs of maxillae, and a pair of poison-feet 
belonging to the first body-segment. In a millipede like 
lulus there are short feelers, mandibles, and a plate-like 
" gnathochilarium " made up apparently by the partial 
fusion of a pair of maxillae. Sir Ray Lankester, who 
strongly supports' the abolition of the class Myriapoda, 
lays very great stress on this difference in the jaws, defining 
the millipedes as " monoprosthomerous " (with only one 
maxillary segment) and the centipedes and insects as 
" triprosthomerous " (with three maxillary segments). 
This distinction cannot, however, be maintained. In a 
study of the jaws of that interesting primitive millipede 
Polyxenus^ I demonstrated the presence of a pair of 
maxillulae in addition to one or two pairs of maxillae. 
Subsequently Miss M. Robinson^ in an embryological study 
of a large luloid millipede has recognised a tritocerebral 
segment in front of, and a maxillular segment behind 
^ " The Structure and Classification of the Arthropoda." Quart. 
Journ. Micr. Sci., vol. xlvii, 1904, pp. 523-582. 
- " Notes on the Segmentation and Phylogeny of the Anthropoda, 
with an account of the Maxillae in Polyxenus lagurus." Quart. Journ, 
Micr. Sci., vol. xlix., 1905, pp. 469-491. 
^ " On the Segmentation of the Head of Diplopoda." Quart. Journ. 
Micr. Sci.. vol, li., 1907, pp. 607-624. 
