376 GEOLOGICAL SURVEY OF THE TERRITORIES. 
brain only innervates the ocelli and eyes, the two pairs of antenne aris- 
ing in Apus from the commissures connecting the supra and infra 
cesophageal g ganglia. In Limulus, the living representative of the Me- — 
rostomata, the first pair of limbs are innervated from the ganglionic sub- 
cesophageal ring, and not the brain; while in most other - Crustacea the 
‘brain supplies the antenna of both pairs, as well as the eyes. Thus, 
apparently, the only sure basis for exact comparison is to begin with the 
first pair of appendages and to regard them, whatever name be applied, 
as homologous throughout the Arthropodan 'series, the parasitic Isopods 
and Copepods perhaps being counted out by reason of the degradational 
changes, which render it difficult to determine in adult life the exact 
homologies of their appendages. 
The general relations of the segments of the bodies of Arthropods 
being similar to what exists in Annelids is to our mind a strong argu- 
ment for the derivation of Trackeate and Branchiate Arthropods, each 
independently, from the worms, the first pair of appendages of the Ar- 
thropods being perhaps homologous with the first pair of tentacles of 
Annelids. 
Homologies of the labrum.—This brings us to consider in passing the 
probable origin and homologies of the labrum* of Arthropods. Weare 
inclined to regard the labrum as possibly the homologue of the median 
frontal tentacle of certain larval Annelids, for instance. If the reader 
will compare Metschnikoft’s figure of the temporary long, large, slender, 
tentacle-like labrum of Cheliter,'the general resemblance to the frontal 
unpaired tentacle of certain Annelids is suggested. We have always 
regarded the clypeus and labrum as a median development, merely 
forming the front wall of the mouth; embryology certainly bears out 
that view. In the embryos of most insects the clypeus and labrum pro- 
ject out remarkably, and may then, perhaps, be compared to the un- 
paired, median tentacle of certain young Annelids. 
The history of this organ is interesting. While in the larval Hstheria 
and Limnadia the labrum is enormous, and nearly as long as the body, 
thus resembling the larval Cirripedia; in the adult it becomes a smail 
fleshy process under the base of the second antenne, and partly ee 
on the base of the mandibles. In Limnetis (Plate XXX], fig. 6, lab.) it 
is rather large. In the Apodide it forms a comparatively ‘large, square, 
horizontal plate (Plate XX XI, fig. 1) on the under side of the head, be- 
hind the frontal doublure. In the Branchipodide it is again reduced to 
a small fleshy inconspicuous lobe. 
The carapace.—This is greatly developed in the Limnadiacec, where it 
forms two large valves, usually with definite “lines of growth,” and con- 
nected over the region of the mandibles by a definite specialized hinge, 
and completely encloses the body, only the second pair of antenne and 
perhaps the telson projecting beyond the edges while the animal is swim- 
ming. Plate XXIV, fig. 9, shows the relation of the bivalvedcarapace 
in Hstheria to the body and its appendages. The hinge has a large 
central median tooth projecting inwards, each valve having a sharp 
denticle which strikes against the central much larger tooth. (See also 
IPD XOXOXO figs 
The histology of the carapace has been described briefly by Grube in 
1865, but his figure (Taf. X, fig. 11) does not express satisfactorily the 
nature of the soft cellular portion or layer. The cuticular layer is strue- 
tureless but laminated, and it has been claimed by Prof. H. S. Morse 
that the carapace valves in Hstheria are due to the fact that the shell, 
‘instead of being cast free from the body when molted, remains attached 
* Hypostoma of Limulus and Trilobites. 
