INVERTEBRATE ANIMALS. 59 



dant on Fuci ; and in Serpula, the meandering tubes of 

 which frequently almost cover the valves of marine 

 shells ; or it may be formed of grains of sand and frag- 

 ments of shells as in Terehella ; or it may be of a 

 leathery consistence, made up of mud strengthened by 

 a glutinous secretion, as in Sabella which at Hilbre 

 may be seen rendering the deeper tide-pools gay as a 

 border of flowers. In Serpula one of the plumes carries 

 a little calcareous lid closing the mouth of the tube 

 when the animal retires. 



II. Order Errantia. In the free Annelides the gills 

 are arranged in tufts along the back or sides of the body. 

 In the Sea-mouse, Aphrodite, the back is protected by 

 scales [concealing the gills, the scales being themselves 

 hidden by bundles of flexible bristles glittering like spicu- 

 lar iron ore, and displaying all the colours of the rainbow. 

 The Lob-worm, Arenicola, burrows in the wet sand, and 

 is much used by fishermen for bait. In the Sand-worms, 

 Nereidee, the anterior extremity is sometimes furnished 

 with eyes, and a mouth with jaws and feelers, and thus 

 makes a nearer approach to a true head than is found in 

 any other of the Vermes. Eunice gigantea attains a 

 length of four feet, and has as many as four hundred 

 segments. The tracks and burrows of the free Annelides 

 on the ancient sea-bottoms are found in rocks of almost 

 all ages from the Cambrian upwards. The tubes of the 

 Tubicola occur as early as the Silurian period. The 

 little Microconchus was found by the writer in the 

 middle coal-measures at Kavenhead. 



