174 AN INTRODUCTION TO THE STUDY OF FOSSILS 



.-ten. 



which is in almost constant move- 

 ment and whose function is rather 

 problematical, though it is doubtless 

 somewhat protective. 



Protection. — Each individual con- 

 sists externally of a chitinous cup, 

 with the broad opening extending 

 outward from the colonial stem. 

 (The material is closely akin to the 

 true chitin of the Arthropoda.) 

 This cup of chitin is the hardened 

 and thickened cuticle of the entire 

 animal except the anterior portion, 

 or introvert (Fig. 69). Into this 

 hard-walled cup muscles can with- 

 draw the soft-walled anterior body 

 portion, while the contraction of the 

 sphincter muscle at the orifice com- 

 pletely closes the cup and protects 

 the inclosed soft parts. By these 

 muscles likewise the tentacles can be 

 swayed in any direction. 



The digestive system is U-shaped 

 with mouth and anus both anterior, 

 and is suspended within the pouch- 

 like coelome or body cavity of the 

 animal. The whole anterior por- 

 tion of the body, the introvert, has 

 walls of thin cuticle continuous with 

 the walls of the chitinous cup and 

 in the contracted state of the ani- 

 mal it is folded back within the cup 

 Hke the turned-in finger of a glove. 

 When, however, the animal expands, 

 the introvert is pushed out through 



Fig. 69. — Bugula avicidaria. 

 Diagrammatic longitudinal sec- 

 tion through a single individ- 

 ual, expanded from its pro- 

 tective cup of chitin, in the 

 position of feeding, an., anus; 

 avic, avicularium; funic, fu- 

 niculus ; g. mus., one of the mus- 

 cles attaching the stomach to 

 the walls of the cup; int., in- 

 testine ; intr., introvert ; loph., 

 lophophore ; mo., mouth ; oe., 

 oesophagus ; ov., ovary ; ph., 

 pharynx; ret., one oi the mus- 

 cles to retract the introvert 

 within the cup ; st., stomach ; 

 ten., tentacles; tes., testis. 

 Much enlarged. 



