METAZOA VERMES. 303 



cally by the animal picking up such materials as it could 

 find and working them over; but the tubes of Serpula 

 (No. 752, S. contortuplicata)) are made by a chemical 

 process and are composed wholly of carbonate of lime. 



In this genus the setae are reduced in size. The gills 

 (see No. 752) are borne on the forward segments of the 

 body and can be withdrawn into the tube, while the open- 

 ing is closed by a tentacle, the end of which has become 

 differentiated into an operculum. 



The habit of living attached and in chemically formed 

 tubes was acquired by some genera very early in geologic 

 times. The Palaeozoic formations yield specimens of 

 Spirorbis which resemble those living to-day (PI. 753, 

 Spirorbis omphalodes Goldf.). Usually in these fossils 

 the tube is hollow throughout, but sometimes the shell is 

 divided into a few chambers by limy walls. Not only do 

 tightly coiled forms occur as represented by PL 753, but 

 some species, like Spirorbis laxus (PI. 754, figs. 1-4), 

 exhibit old age or gerontic characters. The worm on 

 reaching maturity (PI. 754, figs. 1,2) begins to untwist 

 (fig. 3) and in rare cases this process goes on until the 

 whole tube is uncoiled excepting the apical portion (fig. 

 4). It is interesting to note that the round aperture of 

 this tube is smaller than in the mature or ephebic shell 

 (figs. 2, 3) which we have already seen is the case in 

 certain gerontic stages of Cephalopods (see Nos. 617, 

 6 1 8). The descendants of Spirorbis are found living 

 to-day in great numbers. 



The larva of Spirorbis spirillum Gould (PI. 755, figs, i, 

 2), is free-swimming at first, but only for a few hours. 

 It then settles down and begins to build its tube. The 

 tentacles and setae develop early (fig. 3), the former 

 branch rapidly and one becomes specialized into an 

 operculum (fig. 4, /). Thus it is seen that the tendency 

 towards the sedentary habit is inherited so early in the 

 life of the individual that the young Spirorbis (fig. 4) 

 completes its development within the tube. This ex- 



