190 



ANNELIDA. 



as grains of sand, small shells, or fragments of various materials > by 

 means of a secretion which Fig. 77. 



exudes from the surface of the 

 body, and hardens into a tough 

 membranous substance, such is 

 the case of Terebella Medusa 

 (fig. 96). In other cases, as 

 in the Serpula contortuplicata \ 

 (fig- 78), the tube is ho- 

 mogeneous in its texture, 

 formed of calcareous matter 

 resembling the shells of cer- 

 tain bivalve mollusca, and ap- 

 parently secreted in a similar 

 manner. These tubes are ge- 

 nerally found encrusting the 

 surface of stones or other bo- 

 dies which have been immersed 

 for any length of time at the 

 bottom of the sea ; they are 

 closed at one end, and from the 

 opposite extremity the head of 

 the worm is occasionally pro- 

 truded in search of nourish- 

 ment. It must be evident that, 

 in animals thus encased, the 

 character of the respiratory ap- 

 paratus must be considerably 

 modified ; instead therefore of 

 the numerous branchiae ap- 

 pended to the segments of the 

 body which we have found in 

 the Dorsibranchiate order, the 

 respiratory tufts are all at- 

 tached to the anterior extre- 

 mity of the creature, where 

 they form most elegant arbo- 

 rescent appendages, generally 

 tinted with brilliant colours, 

 and exhibiting, when expanded, 

 a spectacle of great beauty. In 

 some species, as in that repre- 



