SKELETONS, SHELTERS AND SPECIAL DEFENCES 655 



ceros, tube production continues from March to September, i.e. during the 

 feeding season. Calcium for tube building is obtained from sea water, and 

 there is some evidence that calcium and radioactive strontium, which is 

 handled in the same way as calcium, are absorbed in the anterior region of 

 the alimentary canal; some part of the calcium secreted by the tubiparous 

 glands is derived from this source (Serpula, MerciercIIa, Pomatoceros) 

 (53, 121). 



Serpulids are capable of dissolving the calcareous matter of their tubes, 

 as well as secreting fresh material. Species of Salmacina and Filograna 

 practise asexual reproduction and new individuals are formed at the 

 posterior end of the parent. The new individual crawls down the parent's 

 tube and makes a fresh opening to the exterior through which it protrudes 



Fig. 15.10. Female Argonaut in Shell 



its crown; furthermore, it may destroy the posterior part of the parent's 

 tube, thus separating it from the rest of the colony. These serpulids may be 

 compared with the sabellid Potamilla, which is able to bore into calcareous 

 shells. The mechanism by which these animals dissolve CaC0 3 is un- 

 known (53). 



Argonaut. The female paper nautilus Argonauta argo dwells in a sym- 

 metrical spiral shell which is secreted by thin terminal expansions of the 

 two dorsal arms (Fig. 15.10). This shell is unique among molluscs in its 

 mode of formation. The shell is also repaired by these same dorsal arms 

 when portions are broken off. The argonaut shell is composed of outer 

 prismatic layers and a middle fibrous layer. Its composition resembles that 

 of other molluscan shells; it consists largely of CaCO s (93% mineral 

 matter), but also contains significant amounts of MgC0 3 (7%) (21, 23). 



Organic Tubes 



Tubes and cases of organic material are formed by many different marine 

 animals, including coelenterates, polychaetes, phoronids, small crusta- 



