Journal of the Proceedings of the Linnean Society - Vol. 3 | Page 2

Brachyura, and not articulated with it as in the Macrura, whilst, on the other hand, the posterior division and caudal termination approach the Macrural type more nearly than that of the Brachyura, the animal thus assuming a character intermediate between the two orders.
But in the description of the several species of the genus Galathea, a peculiar difficulty appears to arise, originating in the affinity which they bear to each other. So close, in fact, is the approximation, that the descriptions of the best writers will scarcely avail for the distinction of the individual species without the assistance of figures. This arises from the fact that the general characters, upon which the descriptions are based, vary, in this genus, only in their comparative degrees of development.
In the three species recognized in Professor Bell's work on the British Crustacea, it will be found that each species retains the same characters in greater or less degree.
Galathea strigosa is peculiar for the spinous character of the carapace and cheliform legs. Every spine, however, is repeated in both the other species, only less developed. We find the rostrum furnished with four lateral teeth on each side, a character which also exists in each of the other species; and although close observation may detect a slightly different arrangement in the relative position of these teeth, the differences are not of sufficient importance to enable a naturalist thence to derive a specific distinction, unless the peculiarity is seconded by some more unqualified character less liable to be affected by any peculiarity of condition.
In order to arrive at more certain results in the identification of species, we think that the microscopic examination of the surface of the integument will be found peculiarly useful.
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. Kr?yii, 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 chel? impinge through their whole length; outer margin of the hand furnished with 3 or 4 small spines.
Hab. Trawling-ground, Plymouth, common; Moray Frith, Scotland.
This species unites G. Andrewsii with G. nexa, and, I think, has often been mistaken for the young of the latter; but G. nexa, so far as my experience goes, is a species peculiar to the north of England, whereas G. dispersa, I anticipate, will be found to be the most universally dispersed, in deep water, of any of the species known.