412 GEOLOGICAL SURVEY OF THE TERRITORIES. 
In his little tract on Worms and Crustacea,! Professor Hyatt refers 
to the simple eyes of Limulus, as if they were the primitive eyes, re- 
tained from larval life. The structure of the two simple eyes of Limulus — 
appears to be in some important respects quite different from that of 
Apus, Estheria, and other Phyllopods, in which there is a circle of 
cones, while in Limulus there is a single large corneal lens on the same 
plan as the facets of the composed eye of the same animal. If, how- 
ever, these simple eyes, be regarded as survivors of the primitive larval 
eye, it would suggest that Limulus and all Merostomata which have 
similar eyes, have like the Neocarida, descended from a Nauplius an- 
cestor; although the development of Limulus polyphemus has been 
shown to be an abbreviated one, the young hatching in the form of the 
adult. The presence of the single eyes would of course be an argu- 
ment for its Crustacean affinities ; while on the other hand the posses- 
sion of compound eyes is a still more important Crustacean character. 
Another point of interest is the mode of moulting in Limulus as com- 
pared with Apus. From our childhood we have found the cast shells 
of Limulus, with the carapace split around the edge of the doablure, 
and we have a partially moulted specimen in alcohol. We have not 
seen a cast skin of Apus, but on asking Dr. Gissler, who has raised the 
young Apus from the egg, as to the mode of exuviation in this Crus- 
tacean, he writes me as follows: “I am certain that the larve of Apus 
(from skins examined) split across or just in front of the eyes, and with 
two or three jerks the animal rids itself of the underlying skin.” It 
would appear then that Apus, which is shaped in front so much like 
Limulus moults in a nearly similar manner. 
In a general way we accept the homologies pointed out by Professor 
Lankester between the Phyllopodous leg and the maxillze and maxilli- 
pedes of the cray-fish, but think that he, in common with Professor 
Huxley, pushes the homologies too far when he proceeds (on p. 365) to 
compare minutely the first leg of Apus with the third maxillipedes of 
Astacus. We do not, as we have stated on p. 391, regard the axis of 
Apus as truly jointed, and he stretches his homologies entirely too far 
when he attempts to homologize the first and second endites of Apus 
with the coxopodite of Astacus; and the third and fourth exites of 
Apus with the basipodite of Astacus. We would suggest that here, as 
among the orders of Arachnida, or Hexapoda, or Myriopoda, if we do not 
stop ata certain point, we are led into erroneous and misleading attempts 
at too close homologies. We should, it seems to us, bear in mind the 
fact that there are ordinal and class homologies; or, in other words, 
there are different degrees of blood relationship, 2. e., different and more 
or less parallel branches of the Crustacean genealogical tree. 
The Decapods did not descend directly from the Phyllopods, but by 
a longer line, independent on the one hand from the Phyllocaridous an- 
cestral line, and on the other from the Branchiopodous stem or branch. 
But a comparison between the Phyllopedous leg and Decapod maxilliz 
and maxillipedes shows that the Decapod exopodite is but a modified 
endopodital lobe, and is not homologous with the exites of the Phyl- 
lopods, the latter corresponding to the epipodite (or gills and flabellum, 
of the Decapods. We have seen that in all Phyllopods the gill and fla- 
bellum are differentiated parts of the epipodal portion of the leg (epipo- 
dite). Huxley’s view, that the base of the corm or “ protopodite” of the 
first thoracic foot is the endopodite, and the endites are merely second- 
1Boston Society of Natural History. Guides for Science Teaching, No. VII, Worms 
and Crustacea. By Alpheus Hyatt, Boston, 1882. 
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