74 HALF AN HOUR WITH SEA-WORMS. 



poses its beautiful plumes. The lower ends of these 

 tubes are usually cemented to stones or shells, for 

 the sake of attachment. Most of these tubed sea- 

 worms creep out of their singular dwellings just 

 before they die, or when they are very sickly, and 

 this fact is useful to those who keep aquaria, as a 

 signal for their removal, and before they can taint 

 the water. We have two species of serpula peculiar 

 to our coasts, that just mentioned and a rarer species, 

 Serpula triguetra. The latter sometimes runs to a 

 great length over the rocks and stones, and is not 

 a very pleasant object for the hand to come in contact 

 with, on account of the sharp, prickly ridges which 

 distinguish it from its fellow. 



These tubed worms have a great geological 

 antiquity. "We find them fossilised in the Silurian 

 limestones, and in every formation since. In the 

 chalk there occurs a species of serpula (jolicata) 

 which is found in contorted masses, running over 

 the naked surface of shells, sea-urchins, &c., just as 

 we find its modern representative thickly clustered 

 on oysters. It thus becomes useful as a sort of 

 chronometrical guide to the rate at which the chalk 

 accumulated over the old sea bottoms. For instance, 

 we know that when the fossil sea-urchins were alive 

 they were covered with spines, so that it would have 

 been impossible for the sea-worms to have then 

 formed their tubes over them. But when the former 

 died, the spines peeled off with the decomposing 

 membrane, and left the surface naked. In this 



