This mode of examination of species may also be applied to a considerable extent throughout the Crustacea generally with great advantage; and if found valuable in recent, there can be no doubt that it will prove of far greater importance in extinct forms, where parts on which the identification of species visually rests are lost, and fragments only of the animal obtainable.
It should be borne in mind that, as the structure in question undergoes modifications more or less considerable in different parts of the animal, it will always be advisable to compare the corresponding parts with each other.
Applying this test to the known species of Galathea, we perceive that the structure of the integument upon the arms, independent of the marginal spines, exhibits a squamiform appearance, but that the scales, which characterise the structure, possess features peculiar to each species.
In Galathea strigosa the scales are convex, distant from each other, smooth at the edge, and fringed with long hairs. In G. squamifera they are convex, closely placed, scalloped at the edge, and without hairs. In G. nexa the scales are obsolete, tufts of hair representing the supposed edges. In G. depressa, n. sp., the scales are broad, less convex than in G. strigosa and G. squamifera, smooth, closely set, and fringed with short hairs. In G. Andrewsii they are small, distant, very convex, tipped with red, and slightly furnished with hair.
As another instance of the practical application of the microscopical examination of the surface, I would refer to two species of Amphipoda, classed by Leach under the name of Gammarus Locusta, from his inability to assign them any separate specific characters. In the structure of their integuments, however, these two forms will be found to exhibit widely different microscopical appearances.
Again, there exists in the same group three or four species, the description of any one of which would apply to either of the others; and it is probable they would never have been ranked as separate species had not their habitats been geographically distant. Thus Gammarus Olivii, M.-Ed., G. affinis, M.-E., G. Kröyii, Rathke, and G. gracilis, R., can only be specifically determined by a microscopic examination of the integument.
The same may be said of other Amphipoda, such as Urothoe inostratus, Dana, from South America, which so nearly resembles in form the U. elegans of the British shores.
Galathea dispersa, mihi.
G. rostro brevi, dentibus 4 utrinque ornato, 2 anterioribus minoribus; pedibus anterioribus elongatis, sparse spiosus; chelarum digitis parallelis.
Galathea with short rostrum, armed on each side with 4 teeth, the two posterior being less important than the two anterior. The fingers of the chelæ impinge through their whole length; outer margin of the hand furnished with 3 or 4 small spines.