METHODS OF ATTACK AND DEFENCE 103 



quite irregular ; Sabellaria, which occurs in immense 

 colonies, makes use only of sand-grains ; Pectinaria builds 

 a very neat tube, in shape like a slightly curved horn, which 

 it carries about with it. The grains are nearly all of the 

 same size and only one layer in thickness. It is among the 

 Amphictenidae that the most finished tube-builders are 

 found ; for instance, the tube of Lagis koreni, abundant on 

 the west sands of St. Andrews, is smaller in shape than that 

 of Pectinaria, but, to quote MTntosh, " in placing the grains 

 together in the tube there is no haphazard, but angle fits 

 angle, as in a skilfully built wall, and no excess of cement 

 hides slovenly masonry. The tube is perfectly round, and 

 tapers from the narrow lower end to the wider upper end, 

 and its symmetry and workmanship are faultless. The 

 tubes harmonise closely with their surroundings, the wall 

 being formed of similar proportions of pale sand-grains of 

 yellow, brown, and black, probably because in such a 

 selection the average colours are fairly represented." 



In Amphictene auricoma the grains of sand are still finer 

 than in Lagis, and where, amongst deep sea mud, the worm 

 is able to employ the spicules of flinty sponges the workman- 

 ship is entirely perfect. 



This habit of tube-building which, originating in the 

 secretion of a simple sheath of mucus to form a lining for 

 the burrow, has become so wonderfully elaborated, is of 

 double importance in the tidal area, for it affords not only 

 ample protection from enemies, but from desiccation as 

 well, the mouths of many of the tubes being closed by 

 opercula which prevent evaporation at low water. 



3. Borrowed Forms of Armour. — It is a wonderful com- 

 mentary on the keenness of the struggle for existence that 

 discarded forms of armour, such as empty molluscan shells, 

 should have been utilised as houses by forms totally unre- 

 lated to the original owner. This is what occurs in the 

 group of the Paguridea, which comprises the Hermit crabs. 

 The habit of lying concealed within a Gasteropod shell is 

 so deep-rooted as to have had profound efltects on both the 

 structure and behaviour of the hermit crab. The abdomen, 



