CRUSTACEANS 



circumstance the sea-shrimp.' ' In this genus the first trunk-legs are so 

 far from being developed into monstrous claws that they were long de- 

 scribed as simple, a technical expression for limbs that have no grasping 

 power. But Dr. Caiman has recently pointed out that they do in fact 

 end in a thumb and finger, though the chela so formed is of only micro- 

 scopical proportions. Very commonly parasitic on this prawn is another 

 crustacean, an isopod called Hemiarthrus abdominalis (Kroyer), which may 

 be discussed at another opportunity. Its occurrence might have been taken 

 for granted, but it is actually reported by Metzger.* 



Not all prawns find salt water a necessity of existence. There are 

 many which are able to live at will also in brackish water or fresh. 

 Among these is Palamonetes varians (Leach), which may be met with in 

 several English counties. The genus is separated from Leander by the 

 absence from its mandibles of the so-called palp. The very same distinc- 

 tion between nearly allied genera is met with in other parts of the class 

 Crustacea, and in some of the instances it is difficult to understand why 

 one set of forms should discard what the other set retains. In Palamo- 

 netes varians^ it should be further observed, the frontal sword or rostrum is 

 straight, entire at the tip, with two teeth on the under margin, and four 

 to six on the upper. Its occurrence in Norfolk is noticed by Bell ; ' but 

 there is, I imagine, an indirect reference to it also in the following argu- 

 mentative and singular record, to which my attention was called some 

 time ago by Mr. Whitaker, lately president of the Geological Society. 

 The passage is in Mr. S. B. J. Skertchly's Memoir on Fenland.* It is 

 headed ' Living Prawns in the Silt.' 



'In the summer of 1873 I investigated a curious case of the en- 

 tombment of the ova of prawns in the marine silt for a lengthy period. 

 Mr. S. H. Miller directed my attention to the case, and accompanied 

 me on my visit. The facts were communicated to Land and Watery 

 and specimens were got which lived for a long time in the Brighton 

 Aquarium. 



' At Walsoken brickyard, near Wisbech, pits are sunk in the clay, 

 and in the year 1859 a bed of fine sand or silt was pierced at a depth of 

 fifteen feet. From this bed a strong salt spring rose, the water of which 

 was much more saline than that of the river in the neighbourhood, and 

 this, mixing with the fresh water in other parts of the pit, rendered 

 it so brackish as to kill the pike, though the carp, tench and insects 

 seemed unaffected by the change. Shortly after this incursion of salt 

 water prawns began to appear in the pit, and the supposition is that their 

 ova were embedded in the marine silt and kept alive by the salt water 

 with which the bed was charged, and that the recurrence of favourable 

 conditions of open water and light enabled them to hatch, and since that 

 time they have abounded in the pits. The largest individuals are about 



' Malacoitraca Podophthalmata Britannia, text to pi. 40, March, 1815. 



* Nordseefahrt der Pommerania, p. 286. 



* British Stalk-eyed Crustacea, p. 310. 



* pp. 241-243, 1877, reprinted in Geol. Survey Mem., sheet 65, p. 135, 1893. 



189 



