34 
Tank No. 22. 
does not 
shells or 
sand in 
adhere to its body. But not all Tube-worms secrete calcareous 
leathery ones, like those mentioned above; some saturate the 
which they burrow with a slimy secretion of their skin and form 
in this way delicate tubes of sand. Such are pos¬ 
sessed by Arenicola, the lug-worm, so. much used 
as bait in England; in the same manner Terebella 
forms a dwelling, and its orange-coloured tentacles 
may be seen protruding from the sand at the bottom 
of the tank, twisting about in all directions in search 
of food. Others cement together small stones, shells, 
and other similar objects (Shell-binders); while some 
encase themselves with mud, or form long horny 
tubes open at‘ both ends and resembling the shaft of 
feather: Onuphis. This latter group crawl about 
Fig. 47. Spirocjr aphis 
Spattanzanii, l j% nat. 
size. 
Fig. 48. Protula in- 
testinum, i /2 nat. size. 
Fig. 49. Serpula uncinata . 
freely like the grub of the Caddisfly, while the "palm-trees” of tank 22 
fix themselves to rocks, woodwork, or other large objects, or simply bury 
the lower end of their tube in the sand. Some kinds are a veritable 
plague in navigation, accumulating in such masses on the keels of ships 
that they retard their progress. 
All these animals, when young, are entirely differently constituted. 
From the eggs issue free-swimming larvae of very curious form, which 
after a while fix themselves to stones or weeds, and then by a complete 
