346 



ZOOLOGY 



SECT 



Occupying the interior of the larva at this stage inaddition to 

 this sac, there is only a mass of undifferentiated tissue derived 

 from the original central tissue together with that derived 



disc 



cor 



cent 



suok 



Fig. 278.— A, Larva of Bugula plumosa ; B, Sagittal section of larva of Bugula (diagram- 

 matic), cent, central tissue ; cor. corona ; disc, retractile disc ; e. ectodermal groove ; p. pyri- 

 form organ ; pall, pallial groove ; such, sucker. (From Korschelt and Heider, after Barrois.) 



from the disintegrated corona, pyriform organ, and part of the 

 sucker. The outer wall forms the wall of the primary zocecium, the 

 surface of which becomes covered with a chitinous cuticle or 

 ectocyst. Most of the internal mass goes to form a brown body, 

 which now becomes developed, but a part of it seems to form the 



mesoderm of the zooid. A diverticulum 

 of the sac constitutes the first rudiment 

 of stomach and intestine ; a second 

 diverticulum forms the rudiment of the 

 oesophagus ; these become applied to 

 one another and fuse to form the con- 

 tinuous alimentary canal. The ganglion 

 arises as an invagination of the ecto- 

 derm in the space between mouth and 

 anus. The upper part of the cavity 

 of the primitive sac, after the rudi- 

 ment of the alimentary canal has been 

 separated off, forms a space termed the 

 atrium ; the walls of this become con- 

 verted into the tentacle sheath, while 

 on its base appear the rudiments of the 

 tentacles and lophophore. During the 

 development of the organs of the adult zooid the brown body 

 becomes closely applied to the stomach and gradually absorbed. 



The primary zooid thus formed gives rise asexually by a process 

 of repeated budding to the branching structure which has been 



Fig. 279. —Longitudinal section of 

 attached larva of Bugula. 

 c. cells of corona ; r. rudiment 

 of the zooid in the form of a 

 sac ; 8. basal' plate of everted 

 sucker ; v. vestibule. (From 

 Korschelt and Heider, after 

 Bari»is.) 



