130 UNIVALVES. 



to it ; the posterior end is thickened and provided 

 with plates which contract the aperture, and 

 render it very small : this part is always even 

 with the surface perforated. Near the extremity 

 of the animal's tail are situated the two lanceo- 

 late valves which seem to perform the office of 

 floodgates, admitting or excluding the water as 

 necessity may require. Linnaeus considered 

 that the valves placed at both ends, were of the 

 nature of opercula, and that the tube was the shell 

 of the animal, which he consequently placed 

 among the univalves. The name is derived 

 from the Greek repsw (tereo) I bore, signifying 

 the supposed manner in which the animal effects 

 a settlement in different substances. 



TEREDO Navalis, 

 COMMON SHIP WORM, 



Specific character. Shell thin, brittle, straight, 

 or flexuous, tapering ; inside smooth, pervious ; 

 the smaller end thick and strong, furnished with 

 plates or laminae, which contract that part con- 

 siderably, and leave a very small opening ; the 

 anterior valves attached to the head of the ani- 

 mal are of a hemispherical form, brittle, thin, 

 finely striated, and covered with a light brown 

 epidermis ; in each there is a long flat curved 

 tooth projecting inwards. The tube is white, 

 sometimes a foot long, seldom so long as the 

 animal ; the foreign specimens exceed greatly 

 in size those found in England. 



