40 REPORT — 1855. 



form, are in G. gracilis represented by minute sharp-pointed ones, which 

 rise out of a soclvet Mhieli lies witliin the tissue itself, and assume the form 

 somewiiat of an iiour-giass, enlarging in diameter as it does at each extremity. 

 Besides these two appearances, there is a third, which, though not present 

 in G. locttsfa, is a feature in the order generally. This is a series of very 

 numerous small perforations, M'hich in some species assume a waved appear- 

 ance as they come through the tissue (PI. XVII. fig. 4). 



Without being confident in the assertion, we think that the object of these 

 tubes is analogous to that of the pores in fish and other marine animals. 



In apposition to the dissimilarity, which often is very great, between the 

 most closely allied species of the same genus, it will not unfrequently be 

 found that the same kind of microscopic structure is repeated in species 

 belonging to genera widely separate. 



In the genus Gammarus, a s])ecies on our shores, which approximates 

 nearer to that of G. Othonis of Edwards than any other of which we are cog- 

 nisant, and has the surface rough, though minutely so, it is sufficient to be 

 appreciable under a lens of low power. When this is examined under a 

 microscope of greater capability, the roughened appearance resolves into a 

 surface irregularly covered with a number of minute projecting obtuse 

 points. These appear to have a tendency to form into rows, the unequal 

 length as well as distance between which are so irregularly repeated, that 

 they appear to exist often together in clusters of greater or less importance 

 (PI. XVII. fig. 6). 



This description of the appearance under the microscope of the dermal 

 tissue in G. Otho-nis (?) would be equally correct of Chelur a terebrans, which 

 belongs to a genus which bears little or no comparative assimilation with 

 Gammarus, the only appreciable difference being that the points which are 

 scattered over the surface of each are perhaps more obtuse in Chelura; but 

 even this may have some modification dependent upon the part of tiie animal 

 from which it is taken, or the relative ages of either (PI. XVII. fig. 10). 



Again, in Dexamine bispinosa of the British seas (which in form much 

 resembles Amphifoe costata of Edwards from the Isle of Bourbon), we see 

 repeated with little variety the same microscopic characters visible in Cal- 

 liope Leachii. In each of these the animal is covered by many small scale-like 

 processes developed upon the surface of the dermal tissue. These, attached 

 at one margin, are raised at the opposite, which is directed posteriorly. In 

 Dexamine there are also present a few solitary small hairs or minute spinules 

 which we have not perceived in Calliope (PI. XVII. figs, 2& 3). 



The scales, broad at their attached base and rounded at the apex, re- 

 semble generally a crescent form in both Dexamine and Calliope. In 

 Dexamine they appear to be more numerous and generally more minute, 

 but it is not impossible that this supposed difference may be dependent upon 

 age or sex. 



Looking at the arrangement of the microscopic structure of the dermal 

 tissue of this order generally, we are forcibly led to rely with con- 

 siderable confidence upon its value as an important test in the diagnosis of 

 species. 



The form and structtire of the hairs which exist on different parts of tlie 

 animal, when microscopically considered, will be found to be auxiliaries of 

 analogous character ; but being not so constant in their peculiarities, are less 

 valuable as tests of species. They not only vary in species, but differ on 

 separate parts of the same animal. In Sulcator arenarius there are no less 

 than twelve varieties. 



1st. Some are plain, simple, stiff, bristle-like spines. These are common. 



