CH. XVI] 



INTERNAL ANATOMY 



377 



the polypide, and the cell the 

 retracted, which occurs when it 

 is irritated, the anterior end is 

 inverted and forms the ten- 

 tacle sheath in which the 

 tentacles lie. The operculnm 

 when present is a movable fold 

 of the body-wall thrown back 

 when the polypide is pushed 

 out, and covering the opening 

 of the tentacle sheath when it 

 is retracted. 



The animal within the cell 

 has a U-shaped alimentary canal, 

 the anus being situated not far 

 from the mouth, but it is sepa- 

 rated from it by a ring of ten- 

 tacles in the centre of which 

 the mouth lies. This crown of 

 tentacles, called the lopho- 

 phore, is not always circular, 

 but may be drawn out into a 

 horseshoe-shaped structure (Fig. 

 178), and in the subdivision of 

 the group (Phylactolaemata) in 

 which it undergoes this modifica- 

 tion there is a small projection, 

 called the epistome, which over- 

 hangs the mouth, being situated 

 inside the tentacle ring. The 

 tentacles are ciliated, and the 

 action of the cilia brings food to 

 the mouth, which leads into an 

 oesophagus also ciliated, and this 

 enlarges into a rounded stomach 

 usually produced into a caecum 

 (Fig. 178). From this a small 

 intestine, parallel with the oeso- 

 phagus, leads to the anus. From 

 cord of mesodermic tissue, called 

 the body wall. 



zooeciurn. If the polypide be 



FIG. 178. View of right half of Plu- 

 matella fungosa, slightly diagram- 

 matic. After Allman and Nitsche. 



1. Lophophore. 2. Mouth. 3. Epi- 

 stome. 4. Anus. 5. Nerve-gan- 

 glion. 6. Oesophagus. 7. Stomach. 

 8. Intestine. 9. Edge of fold of 

 body -wall. 10. Wall of tube. 

 11. Muscles. 12. Funiculus. 



13. Body -wall. 14. Testis. 



15. Testis, more mature. 16. Stato- 

 blast. 17. Ovary. 18. Spermato- 

 zoa free in body-cavity. 19. Ten- 

 tacles. 20. Retractor muscle. 



the aboral side of the stomach a 

 the funicle, usually passes to 



