CRUSTACEANS same interest. Many specimens of the latter were infested, or for some of us invested with an added charm, by the isopod Hemiartbrus abdominalis (Krb'yer), one of their appropriate parasites. The resounding name of this creature is exposed to some cavillings, but until it is displaced it very well suits the facts of the situation, and at least the generic part of it is valid. It means that the animal is half-limbed, in the same way that we call a man half-witted without being too precise as to the halving. The specific title alludes to the fact that this purple-tinged parasite nestles under the pleon or abdomen of the prawn. In Leander serratus there is a very similar and much more familiar species commonly known as Bopyrus squillarum, which lodges in the branchial cavity of the carapace, thereby acquiring for itself a lopsided shape, and giving its host the look of having a swollen cheek. It is rather strange that the Hemiarthrus, though not cramped for room, is nevertheless lopsided even more extra- vagantly than the Bopyrus. The fact is that the female, while still young and slender, catches hold of a front swimming foot of the prawn either with her seven feet on the left or her seven feet on the right, and then allows her opposite side to bulge as it pleases. While all the feet of that side except the first disappear, the marsupial plates develop, some of them, especially the second on the outer side, being greatly extended. Eventually the vast pouch is filled with thousands of eggs. The father of this numerous progeny remains insignificant in bulk. To judge by his dwindled mouth-organs he is no glutton. To judge by his consoli- dated pleon or abdomen devoid as it is of pleopods, he has no inclina- tion to wander from home. He retains his symmetry. His seven pairs of walking legs are undiminished in number. He can therefore, when prompted by a desire for exercise, at least traverse his wife and family, a small domain but his own. Of non-parasitic marine Isopoda, Dr. Sorby reports that Idotea linearis (Linn.) ' occurs at nearly all stations and is usually caught when swim- ming on the surface,' and that a few specimens of /. baltica (Pallas) have been taken under similar circumstances. As to Ligia oceanica (Linn.) he notices that ' in some years this was extremely common on the quays at Row Ledge on the Colne, but in 1900 none were seen.' Of terrestrial Isopoda or woodlice there is no doubt that Essex pos- sesses all the commonest species that are generally distributed in England, but rather singularly the only record is a very recent one, referring to a species hitherto not included in the British fauna. This is Porcellio ratzeburgii, Brandt, reported by Mr. W. M. Webb, F.L.S., from Warley, and identified by the Rev. Dr. Norman, F.R.S. 1 Budde-Lund includes this species in the division of the genus distinguished by the presence of tracheae in the upper branch of all the pleopods, whereas other members of the genus have such spiracles only in the pleopods of the first two pairs. Seeing that crustaceans taken as a whole are essentially water- breathing animals, there is a rather special interest in any modification 1 The Eiiex NaturaRit, vol. xi. p. 127 (1899). I 209 27
