vm PHYLUM MOLLUSCOIDA 339 



of the larva termed the umbrella-shaped stage is reached. The 

 sucker is everted, and by means of it the larva becomes attached. 

 The edge of the " umbrella " becomes bent downwards, and 

 fused with the b^oad plate into which the sucker has ex- 

 panded, thus enclosing a circular cavity, the so-called vestibule 

 (Fig. 284, v.). The walls of this, consisting of the coronal cells 

 and a portion of the original sucker, become broken up and the 

 cavity is merged in the general cavity in the interior of the 

 larva. All the larval structures have now disappeared with the 

 exception of the basal plate of the sucker and the retractile disc. 

 The former gives rise to the basal part of the wall of the primary 

 zooecitun. From the latter, which becomes invaginated, or from 

 a sac which is developed to replace it, are developed both 

 the ectodermal and endodermal structures of the primary zooid. 

 Occupying the interior of the larva at this stage in addition to 

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

 from the original central tissue together with that derived 

 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 FIG. 284. Longitudinal section of 

 separated off, forms a space termed the f^^fr.^SSSt 

 atrium ; the walls of this become con- of the zooid in the form of a 



n '. , , .. sac; s. basal plate of everted 



verted into the tentacle sneatn, while sucker ; v. vestibule. (From 

 on its base appear the rudiments of the figg? and Heider ' after 

 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 

 described. In many of the zocecia of a fully-developed colony no 

 zooid is found to be present, but, instead, there is a dark brown 

 body similar to that which occurs in the primary zocecium. This 

 is a zooid that has undergone degeneration the lophophore 



22 



