376 GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 



brain only innervates the ocelli and eyes, the two pairs of antennae aris- 

 ing in Apus from the commissures connecting the supra and infra 

 cesophageal 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 a^jplied, 

 as homologous throughout the Arthropodan series, the parasitic Isopods 

 and Copepods i^erhaps being counted out by reason of the degradation al 

 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 Tracheate 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. We are 

 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 Metschnikoff's figure of the temporary long, large, slender, 

 tentacle-like labrum of Ohelifer, 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, jjerha^js, be compared to the un- 

 paired, median tentacle of certain young Annelids. 



The history of this organ is interesting. While in the larval Estlieria 

 and Limnadia the labrum is enormous, and nearly as long as the body, 

 thus resembling the larval Cirripedia; in the adult it becomes a small 

 fleshy process under the base of the second antenntie, and partly resting 

 on the base of the mandibles. In Limnetis (Plate XXXI, fig. 6, lab.) it 

 is rather large. In the Apodidm it forms a comparatively large, square, 

 horizontal plate (Plate XXXI, fig. 1) on the under side of the head, be- 

 hind the frontal doublure. In tlie Branchipodidce it is again reduced to 

 a small fleshy inconspicuous lobe. 



The carapace. — This is greatly developed in the Idmnadiacew, 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 antenna? 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 Estheria 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 

 PI. XXXIII, fig. 1. 



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 struc- 

 tureless but laminated, and it has been claimed by Prof. E. S. Morse 

 that the carapace valves in Estheria 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. 



