Page:VCH Kent 1.djvu/312

A HISTORY OF KENT It should be understood that the scuta and terga are two pairs of valves forming the operculum or lid of a Balanus ; the radii are the modified sides of the shell's immovable compartments ; they overlap the alae which are also lateral protuberances of the walls but are overlapped instead of overlapping. Of the strange parasitic Thyrostraca it is quite clear that at least one species has been found abundantly in Kent. This follows from the remarks appended by Bell to his account of the crab Portunus marmoreus^ which he personally appears to have procured only in this county. He says, ' There is another fact relative to this species which is worth recording, and that is, the extent to which they are infested with a remarkable parasite, occupying the space between the folded abdomen and the sternum, and having the prima facie appearance of a bag of immature eggs. Both males and females are equally obnoxious to it, and from its size and situation it must present an insuperable barrier to impregnation. It consists principally of a mass of minute eggs, which are arranged in bundles attached to filaments, like bunches of grapes ; the alimentary canal passes directly through the body, the mouth being attached to the intestine of the crab, which it pierces near its extremity, and from which in all probability it derives its nourishment. The anal opening, which is distinct and obvious, is visible without removing the parasite from its position. The whole is of a rounded trihedral form, and is covered by a tough but thin integument. I have occasionally found it infesting Carcinus maenas, but never in such numbers as in the present species.' ' It is difficult to understand how Bell could have written this account without calling to mind that the parasite which he had observed on the common shore crab had been already described by J. Vaughan Thompson in 1836 as Sacculina carcini. There is now a considerable literature concerned with the structure and life history of the Sacculinidae. It cannot be taken for granted from superficial resemblance that these parasites when found on different hosts belong to one and the same species. The foregoing catalogue of Kentish Crustacea may be deemed a fairly long one and well diversified. It is so. Nevertheless the natural- ist will easily understand that in every direction some, and in many directions very large, amplification of it may be predicted as the result of future researches. » British Stalk-eyei Crustacea, Io8. 262