560 
J. S. KINGSLEY. 
are found in the most primitive group of Crustacea, the Phyllo- 
pods, is against their having much significance. 
The comparison also leads to another correspondence in the 
Crustacea and Acerata, which goes as far to prove its validity 
as did the regional comparison in the case of the Spiders and 
Hexapods. In both Crustacea and Arachnids the segmental 
organs (shell glands of the former, coxal glands in the latter) 
empty at the same point ; at the base of the third pair of 
walking legs in the Spiders and Limulus, and at the base of the 
second maxillae in the Crustacea, or, in other words, at the base 
of the fifth pair of appendages in each. It seems to me that 
this persistence of exactly the same segmental duct in these 
two groups and the absence of any corresponding organs in the 
Hexapods is an argument of no little weight. 
If, however, it be shown that a variation or a disappearance 
of segments or appendages may take place at the anterior end 
of the Arthropod body this comparison will lose part of its 
force. That such obsolescence in the adult occasionally occurs 
is well known, but the presence of the appendages in the 
young readily enable us to check our results. Setting aside 
the parasitic forms the best known cases are presented by 
certain Copepods, Apus, and the Oniscidse, but the fact that 
the normal condition is found in their near relations shows 
that tbis fact is of no morphological importance in this con- 
nection. In Eurvpterus, according to Lankester, there is a 
case of an apparent disappearance of one pair ; but until it is 
proved that the Trilobites are related to the Acerata (a point 
on which, as mentioned above, Walcott’s observations throw 
considerable doubt) the assumption that the cephalic buckler 
of these forms is the exact homologue of the cephalothorax of 
Limulus is not warranted, and in case it turns out that the 
two are different these fossils throw no light on the subject. 
Until it is proven that a segment or an appendage may dis- 
appear in the anterior end of the adults of a large group of 
animals, it is justifiable to assume the exact homology of the 
first pair in all three groups, as I have done above. It is 
possible that an embryological study of the cement glands, 
