AND OTHBR BRITISH PRAWNS; 121 



The condition of the pleopods in the female before and after the 

 breeding-period is shown in text-fig. 5. The shaft or basipodite 

 is broad and flattened on its outer edge into a thin flange, which 

 is either bare or carries a few irregularly-placed setse. Along the 

 inner face of this joint is a series of circular markings which 

 appear to be spots where the cuticle is thin or pierced by a pore. 

 In the centre of some of them is a short conical projection or 

 spine, and the arrangement of these "spinous discs" is definite 

 and apparently almost constant. 



At the moult immediately preceding the laying of eggs a great 

 change takes place. A dense row of feathered setse appears along 

 the outer side of the basipodite and on its posterior surface, being 

 slightly overhung by the thin outer flange. At the same time, 

 on the inner edge of the joint the place of the " spinous discs" is 

 taken by a series of long, smooth setse, while a variable number of 

 shorter setse spring apparently from the lemaining circular spots. 

 These long setae are not the same in number in each pair of legs, 

 but have a remarkably regular arrangement. l"'hey may be 

 described as forming three groups as numbered in the figure, 

 while an additional seta may be present on the anterior face in a 

 position corresponding to the letter "A" in the figure, and may 

 be regai-ded as accessory to Group 3. The following arrangement 

 of these setse was found in a specimen of L. squilla, and may be 

 taken as typical for all the species : — 



Pleopod. 



I. II. III. IV. V. 



Number of setae : Group 1 . . . 4 3 3 — — 



„ 2... - 1 1 - - 



„ 3... 3 3 3 3 — 



„ 3 A... 1 1 1 1 — 



It is to these setae on the inner face that the eggs are attached, 

 and the outer feathered setae are no doubt developed partly as a 

 protection to the eggs, and partly to assist in promoting a current 

 of water through them. 



The presence of these ovigerous setae may be taken as a certain 

 indication that eggs are about to be laid, or that young have been 

 hatched ; while their loss marks the end of the breeding-season. 



Palcemonetes varians var. mesogenitor Sollaud. 



The peculiarities in the reproductive habit of the brackish and 

 fresh- water forms of P. varians are well known, and " physio- 

 logical " varieties have been distinguished, namely var. microgenitor 

 Boas, var. mesogenitor Sollaud, and var. macrogenitor Boas, 

 according to the size of the eggs laid and the stage of development 

 at which the young are hatched. These varieties are, however, 

 said to be structurally indistinguishable. 



