SECT. MI. CRUSTACEA. 203 



have formed. The Limnoria lignorum is particularly de- 

 structive in the harbours on the British coasts, and in the 

 locks of the canals. The tortuous holes it bores are from 

 the fifteenth to the twentieth of an inch in diameter, and 

 about two inches deep. The female Isopod is not more 

 than a line or two in length, the male is a third less, 

 and of a grey or greenish brown. These minute crea- 

 tures bore their holes with their mandibles, which are so 

 sharp and strong that they can penetrate the hardest 

 wood, and appear to feed on it, from the quantity found 

 in their stomachs. Their bodies are covered with pin- 

 nated hairs, their antennae are short, and their posterior 

 end or tail is rounded. 



Most of the genus Cymothea are parasitical ; they can 

 bend the sharp nail of the three first pairs of feet upon 

 the preceding joint, so as to form hooks with which they 

 fix themselves to the fishes on whose juices they feed. 



The Isopods bear a strong resemblance, an almost 

 identity of structure, with the Trilobites, a jointed race 

 of Crustaceans long extinct. Some of the Isopods roll 

 themselves into a ball, as these most ancient inhabitants 

 of the ocean were wont to do ; whose large compound 

 eyes are exactly like those of the Isopods ; whence it was 

 inferred by Dr. Buckland, that neither the constitution 

 of the sea nor the light of the sun had changed for in- 

 numerable ages. The discovery of the Eozoon has 

 proved that Nature has not varied during a period im- 

 measurably prior even to that. 



Entomostraca. 



The Entomostraca form an immense group of the 

 lower Crustacea, consisting of five orders. A vast 

 number are just visible to the naked eye, and many 

 are microscopic ; they teem in every climate along the 

 coasts, and in the deep blue oceans. The horny coat, 



