ANARTHROPODA. 



133 



^>J 



Fig. 84, — Tubicola. a Serpula contoT' 

 tuplicata, showing the branchiae and 

 operculum ; & Sinrorbis communis. 



veniently divided into two groups, according as they secrete for 

 themselves a protective tube {Tubicola), or live a free life (AV- 

 raiitia) ; but the general organisation in these two sections is essen- 

 tially the same. 



The Tubicolous Annelidas or Tube-worms derive their name from 

 the fact that they have the power of protecting themselves by 

 means of tubes (Lat. tvba, a tube ; 

 and colo, I inhabit). In some cases 

 (fig. 84) the tube is composed of 

 carbonate of lime, and is a genuine 

 secretion from the body. In other 

 cases, the tube is composed of 

 grains of sand or pieces of broken 

 shell, cemented together by a glut- 

 inous secretion from the surface of 

 the body. In all the Tubicola the 

 respiratory organs are in the form 

 of branched filamentous external 

 gills, in which the fluid of the 

 pseudohsemal system is subjected 

 to the action of the outer water. 

 They are therefore " Branchiate " 

 Annelides. As they live in tubes, 



however, and do not voluntarily expose more than the anterior end 

 of the body, the branchiae are all placed on or near the head. The 

 filaments of which the gills are composed (fig. 84, a) are richly 

 ciliated, and as the pseudohsemal fiuid is usually red, they have 

 generally a beautiful scarlet colour. 



The most familiar of the Tubicola is the Serpula (fig. 84, a), the 

 contorted and winding tubes of which must be known to every 

 one as occurring on shells or stones on the sea-shore. One of the 

 cephalic filaments in Serpula is much developed, and its extrem- 

 ity forms a kind of conical plug or " operculum," which serves to 

 close the mouth of the tube when the animal is retracted withiji 

 it. In Spirorbis (fig. 84, b) the shelly tube is coiled into a, flat 

 spiral, which is fixed to some solid object. It is of extremely com- 

 mon occurrence on the fronds of sea-weed, and on other submarine 

 objects. 



The so-called " Errant " Annelides (Lat. erro, 1 wander) are so 

 called because of their " roving " habits, the animal leading a free 

 existence, and not being confined in a tube (fig. 85). They have 

 always lateral unjointed appendages, or foot-tubercles, which carry 

 tufts of bristles, and a soft jointed filament or cirrus. The anterior 

 rings of the body are usually so modified as to form a sort of head, 

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