10 



Prof. W. C. M'lntosli on certain 



been free, ])nt otiicrs have l)een immersed in sponges, a 

 situation wliicli favoured the preservation of tlie long external 

 spines. The tube is marked by close wrinkles, which are so 

 fine as almost to be linear, a feature partly due to its compo- 

 sition, for it is formed of a vast number of needle-like glassy 



End of tiil)o of Norwegian TirehvUa. Slightly enlarged. 



spongc-s])iculcs held together by secretion and mud. The 

 spines on the tube are comjwsed of secretion, spongc-spiculcs, 

 and sand-grains, the most ])erfect cxam])lcs [e. 7., those 

 removed from the enveloping sponge) being almost entirely 

 made up of the two former. Another tube, dredged by the 

 * Challenger,' shows longitudinal and somewhat spiral spinous 

 ridges, and is chiefly formed of nnul. 



A firm flexuous tube of sand-grains is constructed by 

 Axionice ; while a large and very lax tube is formed by 

 TereheUa ?/<'/'?/ A^.w, ^I out., on the southern and western shores, 

 but it is generally jirotectod and sheltered by valves of shells 

 (e. g. Fectev) or by stones. ISome show considerable discri- 

 mination in selecting materials for the tube. Thus Sir. I. 



