SKELETONS, SHELTERS AND SPECIAL DEFENCES 



657 



carbohydrate constituents are sugars (glucose, galactose, arabinose, 

 fructose, etc.), acetylglucosamine, glucosamine and glucuronic acid. The 

 presence of hyaluronic acid has been established in integumentary glands 

 of certain species. Older analyses of the horny tubes of certain eunicids 

 (Onuphis, Hyalinoecid) reveal a high content of phosphorus which may be 

 incorporated in the organic matter (23, 25, 26). 



Polychaetes possess the faculty of dissolving the organic material of 

 tubes when occasion demands. When the tube of Sabella is artificially 

 closed, the animal can make a hole in the side of the tube, from which it 

 protrudes its branchial crown. Sabellids are among the few animals which 

 show true phototropism, and are able to bend their tubes so that the 



Ventral 

 sac 



Mucus 

 string' 



Tube 



Fig. 15.12. Tube-building in Sabella pavonina 



Ventral view of the base of the crown and the anterior end of the tube, showing the 

 method by which the string of mucus and sand is applied to the edge of the tube. (From 

 E. A. T. Nicol (89).) 



animal continues to direct its branchial crown towards the light when the 

 direction of the latter is altered. Chaetopterus normally enlarges its U- 

 shaped tube by establishing a new opening at the base of the U, building 

 a new and larger extension and closing off the old arm. It is suggested that 

 in all these instances the animals, by release of suitable enzymes, are able 

 to dissolve the mucoid walls of the tube (36, 90). 



Tubicolous worms frequently possess some mechanism for closing the 

 mouth of the tube when they withdraw. The tube of Hyalinoecia is pro- 

 vided with neat protective valves at either end (Fig. 15.11). The mouth of 

 the tube of Branchiomma and other sabellids usually collapses when the 

 animal withdraws. We have already noted the opercula of serpulids (p. 654). 

 Somewhat similar are the opercula of sabellariids, formed by enlarged 

 anterior chaetae, which effectively block the opening of the tube. 



