IV POLYCHAETA 149 



carry conspicuous branched gills. These may be situated near the middle 

 of the body — about a dozen pairs^ the exact number differing in different 

 species — or they may extend right back to the hinder end of the body 

 as is the case with two species. A remarkable feature of Arenicola, 

 very unusual amongst Polychaetes, is its possession of a pair of otocysts, 

 lying one on each side close to the circum-oesophageal commissure and 

 consisting of a tubular invagination of the outer surface^ dilated at its 

 inner end into a rounded cavity full of fluid and containing little grains 

 of sand which act as otoliths. 



Another very common inhabitant of sandy shores is Terebella which 

 not only burrows in the sand but gives its burrow a certain degree of 

 permanence by lining it with grains of sand, fragments of shell -or even 

 small pebbles, cemented together by secretion produced by conspicuous 

 patches of gland-cells on the ventral surface of the anterior segments. 

 In Terebella there is present a transverse row of long thread-like tentacles 

 projecting from the dorsal surface of the prestomium. Each has a 

 ciliated groove along its ventral side. When building its tube of sand- 

 grains the Terebella stretches out its tentacles over the surface of the 

 sand and grains are caught up and carried by ciliary movement along 

 the groove in towards the head of the worm where they are built on to 

 the edge of the tube. Normally the Terebella extends its tube so that 

 it projects slightly above the surface of the sand, ending off in irregular 

 branched threads of sand which often form conspicuous tufts studding 

 the surface of the sand near low-water mark. 



In the genus Pectinaria also a tube is built up of sand grains cemented 

 together, in this case with great regularity, so as to form a slightly curved 

 tapering " house " which the animal carries about with it. The tube is 

 open at both ends the hinder opening being plugged by the flattened 

 posterior end of the worm and the wider anterior opening being guarded 

 by stout golden chaetae springing in a row from each side of the second 

 segment. 



Finally there exist a large series of Polychaetes which are still more 

 intimately adapted to a tube-dwelling habit ; the parapodia are in these 

 greatly reduced, as is the prestomium with its tentacles, while on the 

 other hand the palps are greatly enlarged, forming branched plume- 

 like and often beautifully coloured gills. In Serpula and its allies the 

 tube is composed of calcium carbonate with a slight organic basis of 

 conchiolin (see p. 268) and a branch of one of the gills forms a conical 

 stopper which plugs the mouth of the tube when the animal with- 

 draws, as it does with lightning rapidity. A very common Serpulid 

 is Pomatoceros triqiieter, which forms the white tubes, with a longitudinal 



