INTRODUCTION. ly 
to detect Cryptothiria, which we have found to be tolerably 
abundant in the genus Balanus, in Cthamalus, whose 
habits and general appearance are so closely allied to it. 
The several genera of the family gide are animals 
peculiarly belonging to the temperate seas, and adequately 
represent the Cymothoide of the torrid zone. It is 
remarkable that, being parasitic upon fishes, no species of 
the latter family has been hitherto detected on our own 
coasts. 
The Asellide flourish chiefly in the temperate regions 
of the seas, being scarcely represented in the frigid zones, 
and not at all in the torrid. 
Arcturus is peculiarly an Jsopod of the colder zones, 
where its species grow to the greatest dimensions in both 
the northern and southern seas; but a single specimen 
has been taken in the torrid zone, in thirty-one fathoms 
of water, north of Borneo. The Jdoteide flourish every- 
where, the largest specimens being in the Baltic Sea and 
near Cape Horn. They live amongst the weed, either 
fixed or floating, and species have been often taken 
swimming free in mid-ocean, where they assume, as 
Crustacea under the same condition frequently do, a deep 
indigo-blue colour. The Spheromide are a family that 
are very littoral in their habits; they range from the 
equatorial latitudes to the colder regions of the temperate 
zones, but die out before reaching the Arctic and Antarctic 
isothermal lines. In hotter latitudes, some species, in 
their depredations on submarine timber, take the place of 
Limnoria, a genus of the Asellide, and surpass it in the 
extent of their capability of injuring submerged wood. 
Ligia, and the other terrestrial genera, appear to find 
their home best in the temperate latitudes, but live from 
the equator to within a short distance of the frigid 
climate. 
These few observations, imperfect as they naturally 
