A HISTORY OF LEICESTERSHIRE 



will yield the Gammarus, its gardens and roadsides and bosky dells will supply 

 many species of woodlice or woodland shrimps. These are the Isopoda 

 terrestria, sessile-eyed, fourteen-legged, with short carapace, in all these 

 respects agreeing with the Amphipoda, but differing from them by having 

 some appendages of the pleon or tail converted into breathing organs, instead 

 of having gills attached to the legs of the middle-body. Their abundance 

 and extensive range make it really singular that they should for so long have 

 escaped all notice in this county, where hitherto, so far as public records are 

 concerned, the only malacostracan rescued from the realms of conjecture has 

 been Asellus aquaticus, Linn. This, indeed, is our only distinctively fresh- 

 water isopod in England, very abundant, very widely distributed, but not to 

 be regarded as quite a typical isopod, since, of the six pairs of appendages 

 proper to the pleon, the female has lost or dispensed with the second pair. 



In contrast with the state of inanition in the great sub-class above- 

 mentioned, the Entomostraca are now making their presence known in no 

 inconsiderable variety. The credit of this development is in a large measure 

 due to two investigators, one being the late Mr. Garnar, whose name has 

 already been brought forward, the other, Mr.J. D. Scourfield, whose intimate 

 knowledge of the subject gives exceptional value to the unpublished list of 

 species with which he has generously supplied me. There are, however, as 

 will presently appear, one or two additional authorities to whom the county 

 is indebted for discoveries of special interest. 



The three orders among which the Entomostraca are distributed, the 

 Branchiopoda, Ostracoda, and Copepoda, are so remarkably unlike in general 

 facies that some study is required before the propriety of classing them side by 

 side can be appreciated. Yet, in spite of this diversity, there are certain 

 forms which have been bandied to and fro between the first and third orders, 

 without obtaining, even to this day, a quite secure position in either. These 

 are the Branchiura, at present by some accepted as a sub-order of the 

 Branchiopoda, sharing that rank with the Phyllopoda and Cladocera. Alter- 

 natively, the three divisions have been raised to independent orders, near to 

 one another. But Mr. Charles Branch Wilson, in his very valuable 

 ' Systematic Review ' of the family Argulidae, 8 would again make them a sub- 

 order of the Copepoda. This is not a fitting opportunity for discussing his 

 arguments. Apart from these his definition of the Branchiura may con- 

 veniently be quoted. According to this they have a ' flattened body, consisting 

 of a shield-shaped cephalothorax in which the first thoracic segment is fused 

 with the head, a free thorax of three segments, and a two-lobed abdomen 

 without segments ; four pairs of swimming feet, long and furnished with two 

 rows of plumose setae ; two large compound eyes, movable, and surrounded 

 by a blood sinus ; testes in the abdomen ; heart present ; females without ovisacs, 

 eggs attached to foreign objects.' There is only one family, including three 

 genera. 



For instituting a comparison between the Branchiura on the one hand 

 and the malacostracan crayfish or isopods on the other, we may accept the 

 opinion that in the former the paired appendages, apart from the eyes, repre- 

 sent in succession first and second antennae, mandibles, first and second 

 maxillae, maxillipeds, and four pairs of two-branched locomotive limbs, in 



* Proc. U.S. Nat. Mas. xxv, 701 (1902). 



