CRUSTACEANS 



cheeks of the carapace. In the tail-segments or pleon it is not very difficult to trace similarity 

 of character throughout the Malacostraca, but the appendages of these segments are modified 

 in a strange variety of ways to serve different functions. In the higher forms some of them are 

 altogether dispensed with, just as we find the proudest of the mammals dispensing with their 

 caudal vertebrae. In that division of the Amphipoda to which Gammarus pulex belongs, the first 

 three segments of the pleon are always articulated and carry appendages called pleopods 

 or swimming-legs, with a function corresponding to their name. Their structure is tolerably 

 simple, consisting of a two-jointed stem and two many-jointed branches. Even when 

 the animal is stationary their movements are not entirely discontinued, being no doubt 

 necessary for maintaining a proper flow of water over the branchial vesicles. The foremost 

 pair is often found directed strongly forwards. In the female the object of this position is to 

 assist in keeping the eggs or developing young, safe within the marsupial plates. In Gammarus 

 and its neighbours the fourth, fifth, and sixth pairs of pleon-appendages are known as uropods 

 or tail-feet. The last pair may be used as a steering apparatus, but sometimes all three assist 

 the animal in springing movements by which the rowing action of the pleopods is supplemented 

 or superseded. 



The Isopoda are represented here as in other inland districts of our country by a single 

 freshwater species, Asellus aquaticus (Linn.), noted by Professor Carr as 'common in similar 

 situations' 1 with G. pulex. Many fine specimens of this species were sent me by Mr. H. V. 

 Machin, of Gateford Hill, Worksop. They reached West Ealing opportunely for the 

 commissariat of the earlier arrived crayfishes. Much to my surprise, when introduced to one 

 another the crayfishes made not the slightest attempt to catch or molest the Aselli, and these 

 on their part swam and crawled about not only within reach of the claws, but close to 

 the mouths of the crayfishes. Their immunity was not permanent. But my impression is that 

 crayfishes prefer to take their meals in the dark, and do not much care for game, at least in the 

 guise of earthworms and water boatmen, until it has been kept a decent time. For a speci- 

 men of Asellus aquaticus from a shallow well at Chilwell I am indebted to Mr. Charles E. 

 Pearson, F.L.S. 



The Isopoda agree with the Amphipoda in having the eyes not stalked but sessile, and in 

 having the seven segments of the middle-body or person freely movable, and not covered by 

 the carapace. On the other hand they differ strikingly in regard to the breathing apparatus, 

 which is in them transferred from the walking legs to the appendages of the pleon. This 

 carries with it a transfer of the heart from its position near the head to a position near the tail. 

 Like Potamobius pallipes among the Macrura, Asellus aquaticus among the Isopoda is a convenient 

 object of study because of its extreme commonness. But whereas the former species is in most 

 respects normal to an exemplary degree, the Asellus is very abnormal in the arrangement of the 

 pleon. Not merely are all the segments of this part consolidated into a single shield, 

 but beneath this sort of carapace the appendages are eccentric, differing in number in male and 

 female, and some of them rather widely differing in shape in the two sexes. 



Of terrestrial Isopoda the species known to occur in England are now twenty-four in 

 number. When Bate and Westwood published their work on British Sessile-eyed Crustacea 

 in 1868 they were only able to record sixteen of these twenty-four, and for Nottinghamshire, 

 Professor Carr's list in 1904 contained only seven. A collection kindly made for me by 

 Mr. C. E. Pearson during the winter of this present year, 1905, enables me to make one 

 addition to the latter number. These eight species are distributed over three families of the 

 Oniscidea. All of them are commonly known as woodlice. This term, it may be said 

 in passing, is quite unworthy of their true carcinological rank. They are as much Crustacea 

 Malacostraca as any crab or lobster that ever was eaten. In the family Trichoniscidae stands 

 Trichoniscus pusi/lus (Brandt), described by Bate and Westwood 3 under the name Philougria 

 riparia (Kinahan). Professor Carr says of it, ' This tiny claret-brown coloured species is very 

 common under stones, decaying logs, amongst moss, etc., in damp places. I have found it more 

 or less abundantly at Basford, Cinder Hill, Wollaton, Kimberley, near Bulwell Wood Hall, at 

 Kirkby in Ashfield, Mansfield, Warsop, Nether Langwith, Creswell Crags, Shireoaks, and 

 Worksop.' 8 The animal is only a sixth of an inch long, with very minute eyes, the 

 second antennas much geniculate, and the first pair, as usual in this terrestrial group, very small. 

 The pleon is abruptly narrower than the middle body, and, as in almost all the Isopoda, has 

 the terminal segment or telson fused with the preceding segment. This terminal piece has 

 here a truncate apex, distinguishing it from the corresponding part in the next five species, 



1 Op. cit. p. i. 8 Brit. Seti. Crust, ii, 456. s Trans. Nott. Nat.Soc. for 1902-3, p. i, 



I 145 19 



