84 



genera, into two families, Ligicz and Onisci, distinguished by- 

 various anatomical details, the most distinct of which are the form 

 of the uropods and the number of hairs on the inner plate of the- 

 maxillulae — three in the Ligice and two in the Onisci. The 

 anatomy of the terrestrial Isopods has been fully described in 

 various zoological w r orks and is, of course, very interesting for 

 comparison with that of the marine species as showing the modifi- 

 cations of structure caused by a life on land. For one thing, all 

 the mouth-parts are reduced, and, so to speak, solidified: all the 

 long hairs so characteristic of the mouth-parts of the aquatic 

 Isopods, where they serve to direct and filter a current of water^ 

 are absent- The mandibles are large and strong, while the 

 maxillipeds are fused in a somewhat remarkable manner. Then 

 the breathing apparatus is, of course, quite changed as that of the- 

 marine and freshwater species is entirely adapted for an aquatic 

 existence, and in certain of the woodlice the need for some other 

 method of obtaining oxygen is supplied by a system of " pseudo- 

 tracheae " in the pleopods. These bear a considerable likeness 

 both in appearance and function to the tracheae of the insects, and! 

 consist of ramifying tubules filled with air and opening to the 

 exterior by slit-like apertures. It is interesting to note that these- 

 pseudo-tracheae do not occur in the Ligiidce or more primitive 

 types, and increase in importance as they appear in the more 

 specialised genera. One of the most primitive woodlice is the- 

 " Quay-louse," Ligia^ while the form most adapted for a 

 terrestrial life is possibly the familiar Armadillidium, which is an 

 example of extreme specialisation. Its habit of rolling itself into 

 a ball when alarmed (in which it seems to have advanced along 

 the same lines as the " Pill-Millepede") ;, its reduced antennae and 

 unobtrusive uropods and short legs ; its power of existing in a dry 

 atmosphere and its advanced pseudo-tracheae all make it fit in the 

 highest degree for a life upon land. 



As with so many groups of Crustacea, it is difficult either to 

 arrange the woodlice in any definite and scientific order of 

 relationship or to find the connection between them and the other 

 Isopoda. Certain aberrant foreign genera, Helleria and Tylos, 

 have even been looked upon as terrestrial members of the- 

 sub-order Valvifera, and various attempts have been made to show 

 the relationship of the Oniscoidea to this and other groups, but 

 the matter needs much fuller investigation than it has so far 

 received. 



There are in all, I believe, some 34 of the Oniscoidea 

 recorded from the British Isles, of which number I have taken 18, 

 or just over half, from our district, and I have no doubt that a 

 thorough search would result in the discovery of three or four- 

 more species in this neighbourhood. The other British species- 

 are mostly very local in distribution, and are not likely to be taken 

 here. Only six or seven species appear to have been previously 

 recorded from Hampshire, and I have, therefore, thought it 

 advisable to prepare a list of species which I have taken here,. 



