ORDER CRUSTACEA HETEROBRANCHLE. 139 



flat ; it is essentially a dweller on the land, but it cannot 

 exist except in damp places, where the moisture is 

 sutncient to keep its branchis3 pliable ; it belongs to a 

 group of small Crustacea known by the name Oniscus. 

 Some, as we see in the present instance, frequent the 

 land, but the greater portion inhabit the water. 



Cloportus asceUiis. (Fig. 1, much magnified; fig. 2, natm-al size.) 



Among those which inhabit the latter element, there 

 is a minute species which is very injurious to timber. 

 It excavates a cylindrical hole for its dwelling, and 

 increases in number so rapidly, that in a few years 

 timber which is covered with water is rendered useless. 

 The temporary wood-work used during the time the 

 Bell-Rock Lighthouse was in the course of erection, 

 was destroyed, to a great extent, by this httle creature 

 "When the wood had been under water for three years, 

 beams ten inches square were reduced to seven inches ; 

 at the rate of one inch a year. Another species, Cymo- 

 thoa, attaches itself to the backs of different species of 

 fishes, living upon the juices of their body. 



A crustaceous animal nearly aUied to this last is de- 

 scribed in the fifth volume of the American Philoso^ 



k2 



