INTRODUCTION. Iv 



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 ^Egida are animals 

 peculiarly belonging to the temperate seas, and adequately 

 represent the Cymothoidae 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 Asellidce nourish 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 Isopod 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 Idoteidce 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 Sphceromida 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 Asellidce, and surpass it in the 

 extent of their capability of injuring submerged wood. 



Liffia, 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 



