118 LIFE BY THE SEASHORE. 



blance to a Sabellid, and also the character of the 

 operculum. 



Two other common Serpulids may be named without 

 description. These are the tiny Spirorbis, which forms its 

 coiled white tubes on Fucus, and is often very abundant, 

 and Filigrana implexa, a social form whose narrow, inter- 

 lacing tubes are often very conspicuous on the shore rocks. 



The last worm we shall describe is Sabellaria alveolata, a 

 curious and interesting form often very abundant on the 

 shore. It is not closely related to the preceding worms, 

 and forms a very firm but irregular sandy tube. These 

 hard tubes are sometimes found singly on shells and stones, 

 but in places where the worm really flourishes, numbers of 

 tubes occur together, so that the worms build up blocks of 

 what looks like coarse porous sandstone. These blocks are 

 hard, and the worms delicate and fragile, so that it is by no 

 means easy to obtain perfect specimens. 



Before examining worms removed from the tubes, watch 

 some uninjured specimens within their tubes. They will be 

 seen to protrude from the tube a crown of bristles, similar to 

 those of Pedinaria but less brilliant, and also numerous 

 tentacles. The tubes differ, however, from those of Pectin- 

 aria in being quite immovable. 



In the specimens removed from their tubes notice that 

 the body is sharply bent, so that the posterior region with 

 its terminal aperture lies at the opening of the tube close to 

 the mouth. The worms are not more than an inch long, and 

 the anterior thickened region is usually of a bright purplish 

 tint, while the narrow reflexed posterior region is paler in 

 colour. The peristomium has grown right forward over the 

 head and bears the prominent bristles. The prostoinium, 

 as in Sabellids, bears numerous gill filaments, but in addi- 

 tion there are dorsal cirri which act as gills (cf. Terebellids). 

 There are many other structural peculiarities too difficult to 

 be discussed here, but the hardened masses of tubes, the 

 purple colour, and the peculiar shape are so characteristic 

 that there is little difficulty in recognising the worm. 



In concluding this survey of the bristle-worms it may be 

 well to point out that their great abundance makes it very 

 difficult to mention more than a few representative forms. 

 They occur everywhere on the rocks, and are adapted for all 



