CRUSTACEANS 



directions, the outer maxillipeds in oscillating movement, the tail-fan 

 fully expanded, forward it strides.' 1 Commonly the two large claws or 

 chelipeds seize the food, and the two following pairs of little claws tear 

 it in pieces and pass it on to the mouth organs. Not every one perhaps 

 will have noticed that in lobster and crayfish alike there are three pairs 

 of claws, or will have reflected on the great advantage which the animal 

 derives from having them of different sizes. Not only are the small 

 ones much more conveniently disposed than the great raptorial pincers 

 for conveying morsels of food to the mouth, but by reason of its small- 

 ness the chelate ending of these limbs does not interfere with the ambu- 

 latory function which they also have to fulfil. For purposes of classifi- 

 cation we speak of five pairs of persopods, walking-legs, or trunk-legs, 

 throughout the Malacostraca, but in function some of them exhibit many 

 modifications. In many species, as in that under discussion, the first 

 pair are rather hands than legs, while, as just observed, the two following 

 pairs are hands and legs at the same time. The crayfish can walk for- 

 wards, backwards, or sideways, in water or on land, though more awk- 

 wardly on the latter than in the former. How these objects are severally 

 attained by co-ordinated muscles, special modes of articulation, and rela- 

 tive lengths of the limbs, is fully explained in Dr. List's treatise. To 

 one important relation, by way of example, may attention here be 

 directed. In the four hinder pairs, the proper walking-legs, there is not 

 uniformity of action, but a kind of antagonism. In the forward move- 

 ment the force of the first three pairs acts as a pull, whereas that of the 

 fourth pair plays the part not of a pull but of a push. Accordingly, 

 Dr. List points out, in Huxley's Crayfish the frontispiece, though in 

 general an excellent and carefully drawn figure, represents the animal 

 with its feet in a not very natural position, since all the legs are pointing 

 in the same direction. It is not unlikely that the picture was drawn 

 from a ' specimen ' rather than from life, so that the artist had no means 

 of knowing that the forward-pointing toes of the last legs ought to have 

 confronting them those of the three preceding pairs. 



Turning now to the less conspicuous group of the terrestrial Iso- 

 poda, the woodlice, we see by a striking instance that the carcinology of 

 this county, though in appearance very unpromising, is beyond all reason- 

 able doubt potentially rich. About the distribution of these small 

 obscure crustaceans in this district little was known, and nothing pub- 

 lished, almost down to the close of the nineteenth century. It then 

 happened that the Rev. Canon Norman, F.R.S., an acute and trained 

 observer, removed from the north of England to a residence in Berk- 

 hamsted. An instructive result was speedily forthcoming. Ten species 

 of Oniscoidea were recognized by Dr. Norman as belonging to Hert- 

 fordshire. These he found, not by traversing and exploring the length 

 and breadth of the county, but all of them in his own garden. The 

 complete muster roll of species of this group, as at present definitely 



1 List, ' Morphologisch-biologische Studien flber den Bewegungsapparat der Arthropoden,' in 

 Morphol, Jahrbucb, vol. xxii. p. 412 (1895). 



