ON THE BEITISH EDRIOPHTHALMA. 39 



forms of Crustacea, but the tissue is never consolidated into so firm a struc- 

 ture. It seldom, except in the larger species, and in certain parts of others 

 where strength is required, as the chelae, &c., increases to such an extent as 

 to cease to be transparent. This circumstance offers to the observer very 

 valuable advantages. Without necessarily destroying life, one is enabled to 

 perceive the currents of the circulation of the (so-called) blood ; also the 

 motion of the cardiac vessels, and the position of many of the internal organs, 

 which otherwise could never be clearly ascertained ; since in the dissection of 

 an animal so small, a great disarrangement of the tissues must necessarily 



take place. . i j i 



Independent of the advantage of being able to see through the dermal 

 tissue, we are also capable of examining its minute composition, and the 

 manner in which it is built up, without cutting the material into thin sec- 

 tions, and thus precluding the examination of its character as a whole. The 

 examination of this tissue microscopically is one of considerable im- 

 portance, as we believe it will be found to offer very extensive varieties of 

 structure, the extent of which is limited only perhaps by the number of 

 species in the genera ; for as far as our examination has progressed, we have 

 found the law of peculiarity of structure constant to every species, a cir- 

 cumstance in itself of great advantage in the determination of doubtful 

 specimens. 



Although a great dissimilarity of the microscopic structure between spe- 

 cies belonging to the same genus is persistent to such an extent, as to differ 

 ■widely even when the general appearances of animals assimilate so that they 

 may be mistaken otherwise for the same species, yet we find that in different 

 genera the character of the structure of the dermal tissue is repeated with 

 but little modification ; as compare Gammarus ( Othonis ?) with Chelura 

 (Plate XVII. figs. 6 & 10), also Dexamine with Calliope Leachii (figs. 2 & 3) 

 in the same table. 



The closely allied species, which by Leach in his typical collection in the 

 British Museum are arranged under the same head as Gammarus locusta, 

 will be found, in spite of the very near resemblance in external character, to 

 have a considerable variation in the microscopic appearance of the integu- 

 mentary tissue, and are in fact two species, G. locusta and G. gracilis. 



In Gammarus locusta the dermal skeleton will be found, when examined 

 under one-fifth of an inch power object-glass, to possess a minutely granular 

 appearance in its general aspect, studded here and there with small short 

 arrow-headed spinules or hairs, around each of which is a semitransparent 

 areola, it being free from granular material. In addition to the arrow- 

 headed points, which at intervals cover the general surface, there is in this 

 species on each side of the medial line of the four or five posterior seg- 

 ments of the (so-called) thorax, a row of small simple-pointed spines: these 

 are closely placed together to the number of nine or ten in a semitransparent 

 areola which surrounds the entire set; the whole arranged in the form of a 

 short, rather abruptly curved line (Plate XVII. fig. 5). 



The closely allied species we believe to be identical with Gammarus gra- 

 cilis of Rathke, and perhaps also G. Olivii and qffinis of Edwards, but which 

 only a microscopic examination of the structure of the skin could positively 

 determine, since they have been found at very distant habitats ; the former 

 at the Crimea, the latter at Naples. In this species, the most abundant 

 upon our shores, the granular pavement is not so conspicuous; the walls 

 of the cells, of which the tissue is constructed, are still apparent in their 

 general arrangement. They form polygonal divisions caused by their mutual 

 pressure. The small spinules, which in G. locusta assume an arrow-headed 



