656 THE BIOLOGY OF MARINE ANIMALS 



ceans and lamellibranchs. These shelters vary greatly in shape and com- 

 position. A burrowing sea-anemone (Cerianthus) lines its burrow in the 

 mud with mucoid material strengthened by nematocysts. Some amphipods, 

 such as Amphithoe rubricata, live in tubular nests formed of weed frag- 

 ments held together with threads secreted from pedal glands. A few lamelli- 

 branchs, e.g. Lima, build nests of byssus threads. Tubes of phoronids have 

 an organic base containing chitin, and are leathery, impregnated with cal- 

 careous material or strengthened with sand grains (6\b, 79, 136). 



Tubicolous animals, par excellence, are polychaetes, many of which 

 secrete tubes of so-called mucoid material. Mucoid tubes vary in con- 

 sistency from soft jelly {Myxicola, Flabelligera) to leather- or horn-like 

 consistency (Chaetopterus, Hyalinoecia) (Figs. 4.2, 5.3 and 15.11). Fre- 

 quently, foreign materials are deposited on a mucoid base, e.g. sand grains, 

 mud and fine gravel in tubes of Lanice, Branchiomma, etc. (Fig. 15.9). 



(a) 



Fig. 15.11. Tube of Hyalinoecia tubicola 



(a) Entire tube ; (b) two pairs of valves at anterior end of tube in closed positions 

 (c) posterior end of tube, valves open (above), closed (below) (after Watson, 1903). 



Arenaceous tubes of some sabellariids may be aggregated together in such 

 dense masses as to constitute sandstone reefs (Sabellaria, Gunnarea). 



It is generally agreed that mucoid tubes are secreted by epidermal 

 glandular cells, widespread {Myxicola), or concentrated in certain regions 

 which mould the form of the tube, e.g. epidermis of peristomial collar and 

 thoracic membrane of Sabella. The process of tube formation has been 

 described in several species. In Sabella pavonina a. sorting mechanism 

 exists on the gill filaments whereby medium-sized particles are carried by 

 ciliary activity to ventral sacs lying below the mouth. Additions are made 

 to the anterior end of the tube with material stored in the ventral sacs 

 (Fig. 15.12). The particles are mixed with mucoid material into a string; 

 by rotation of the anterior end of the animal and moulding action of the 

 collar-folds the string is laid along the edge of the tube and cemented into 

 place with mucoid secretion (89). 



The mucoid constituents of polychaete tubes are variants of glyco- 

 proteins (polysaccharide-protein complexes). Mucoproteins have been 

 identified in tubes of Myxicola, Sabella, Spirographs, Lanice, Pectinaria, 

 Hyalinoecia et al. Acid hydrolysis yields the common protein amino-acids ; 



