102 THE BIOLOGY OF THE SEA-SHORE 



than " spillikins." The larger of these, and particularly the 

 stout forms known as acicula, are probably of some use as 

 weapons either of offence or defence ; but in any case the 

 occurrence of so many bristles must render the owner very 

 unpalatable as food. In Polynoids the bristles are sup- 

 plemented by flexible, overlapping scales which protect the 

 back. In Aphrodite aculeata, bristles and elytra form a dense 

 felting which must not only provide valuable protection, but 

 by collecting mud, etc., also serves to merge the animal into 

 its surroundings. 



2. Construction of Tubes. — This is a characteristically 

 Annelid habit, though occasionally forms which construct 

 tubular dwellings are met with in other groups also. The 

 forms of Annelid tubes have been discussed in an interesting 

 manner by MTntosh (1894). In its simplest phase, e.g. in 

 many Oligochaets, the tube is simply a thin envelope of 

 hardened mucus, and is little more than a lining to a tem- 

 porary burrow. As a development of this we have the more 

 permanent tubes of forms such as Myxicola which are soft 

 and gelatinous, or the Eunicidae which are parchment-like 

 or even leathery. In Hyalinoecia the material secreted, 

 consisting of a phosphoric salt containing magnesia, and an 

 organic substance called " onuphin," is so firm that the tube 

 is able to stand freely in the mud and the animal has no 

 need of further protection. In many worms the body 

 secretion is of a calcareous nature and forms on consolidation 

 a hard shelly tube. The commonest of these are Serpula, 

 Pomatoceros, and Spirorbis. Probably the most numerous 

 forms of all are those in which the secretion is used for the 

 purpose of cementing together particles of mud, sand, 

 gravel, shell, etc. Some of the tubes so formed provide 

 examples of animal workmanship so accurate and beautiful 

 as to challenge comparison with the combs of bees and wasps 

 and even with the nests of the most skilful birds. The tube 

 of Sabella consists of the very finest particles of mud ; 

 Terebella conchilega makes use of shell-fragments, grains of 

 sand, spines of heart urchins, etc., no special regularity 

 marking their arrangement. The tube of Nicomachia is also 



