286 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



into it. In the Animlata it often becomes of con- 

 siderable thickness and is then traversed by pore-canals. 

 In the sedentary marine Annelids a tube is developed 

 as a means of protection ; the inner portion, which is 



partly membran- 

 ous and partly 

 fibrillated, is 

 formed by special 

 glands in the 

 body wall ; out- 

 side this the tube 

 is often rendered 

 moreresistent by 

 the deposition of 

 calcareous 

 matter (as in 

 Serpula), or of 

 aggregations of 

 sand, mud, and 

 other foreign ma- 

 terial (as in Sa- 

 bella, or Amphi- 

 trite), which are 

 taken up by the 

 tentacles of the 

 worm, and laid 

 down on the tube 

 by the animal it- 

 self. Within this 

 tube the inhabitant may be retracted, and some (as 

 Sabella) form an operciilum by means of which the 

 entrance to it may be closed. 



In the Sabellidse special cartilaginous supports are 

 developed within the gill tentacles ; this is not found 

 in the Serpulidse. 



The cells of the integument often give rise to hard 

 projecting structures, which may have the form of 



Fig. 119. Gelatinous Tissue from the Disc of 

 Aurelia aurita; a, fibres: b, cells. (After 

 M. Sclxultze.) 



