ON THE BRITISH BDBIOPHTHALMA. 4f 



peduncle ; for the two appear to be so closely associated, that it is impossible 

 to say to which it more immediately belongs. From analogy with the higher 

 types, we should infer the first, though probably the two combine to increase 

 the efficiency of the organ by their concentration. 



In the freshwater species of Gammarus, the organ appears rather larger 

 and more characteristic in form. It is from this species we shall give our 

 description of the organ. 



The first joint of the antenna is enlarged into a chamber of a globose form 

 (PI. XIV. fig. 4a) : this is received into a corresponding notch of the cephalic 

 ring (fig. 3). From the globular chamber, which appears to be the pro- 

 tecting walls of an internal organ of more delicate contrivance, there proceeds 

 a large tooth-like process (b), which in this Report we have called the olfactory 

 denticle. It differs in length and breadth in different species, but is a very 

 constant appendage. This process is open at the extremity (c), through 

 which a tube projects (d), which latter is either open, or protected by a 

 membrane too delicate to be observed, but which, from analogy with the 

 higher orders, we are induced to believe may be the case. It is not always 

 that the tube projects through the aperture at the extremity of the denticle; 

 occasionally it falls short, as in Istea (fig. 1); but this is merely a variety 

 depending upon species. 



The tube appears to be cylindrical, and continues internally with parallel 

 walls to about half the length of the tooth itself, when it suddenly converges 

 to a point, which is open, since it is entered by what appears to be a nerve, 

 which either itself terminates in or supplies with sensibility a sharp tongue- 

 like process (/), which is enclosed within the cavity of the tube-like canal. 

 From the base of this small organ the supposed nerve is traceable in a waving 

 line to a small bulbous origin (ff), situated at the base of the olfactory 

 denticle at its point of connexion with the enlarged chamber. Beyond this 

 probable ganglion the closest investigation has not enabled us to see any 

 further trace of the nerve. 



This organ, with but little variation of external form, is to be met with in 

 almost every species, even including those where the whole antenna is pro- 

 duced in the form attributable to the character of Ieg«, and used as such in 

 climbing over irregular protuberances of the ground. 



The species in which the organ in its external form does not exist, are the 

 Talitri, Orchestia, and the Hyperice, together with a species of Gammarus, 

 which we believe hitherto to be uudescribed ; we call it in our list Gammarus 

 elegans, on account of the general beauty of the form and colouring of the 

 only specimen we have yet taken *. The lower antenna in this species is sup- 

 plied with a peculiar set of organs, similar to those which have been described 

 by Prof. Edwards in his species G. ornatus. Commencing on the last joint of 

 the peduncleto the extremity of the long filament, there is, at gradually increa- 

 sing intervals, a series of small membranous polyp-like bodies : they are closed 

 sacs, and require but a low power of the microscope to perceive them. Those 

 described by Edwards are fringed with a slightly ciliated border, and belong 

 to a North American species, which differs in other essential respects from our 

 British form. To assign any peculiar use to these organisms came not within 

 the conception of their original observer, and we can only point to this solitary 

 instance of their being present on the olfactory antenna, where the organ of 

 the sense peculiar to it is either absent or reduced to a rudimentary cha- 

 racter: but a more extended opportunity of observation is necessary before 

 we can attempt to pronounce this condition constant (PI. XIV. tigs. 5 & 5a). 



* This may be the true reason why the olfactory denticle has not been observed : we 

 were »&aid of injuring the specimen. 



