338 



ZOOLOGY 



SECT. 



fill the blastocoele ; this mass apparently represents both 

 endoderm and mesoderm. Small cavities which appear in it 

 subsequently unite together to form the primitive coelome. 

 A very broad ring-shaped thickening the corona (G, cor.) 

 is formed round the equator of the embryo and becomes 

 provided with cilia. A circular pallial groove arises on the 

 oral side of the corona. A sac-like, afterwards beaker-shaped 

 invagination of the ectoderm on what is destined to become 

 the oral side of the ciliated ridge, forms a larval structure, termed 

 the sucker (Fig. 283, suck), which afterwards serves to fix the larva. 

 A second depression of the ectoderm in the region of the corona 

 on the oral side forms the ectodermal groove. At the aboral pole is 

 developed, also from the ectoderm, a second larval structure the 

 calotte or retractile disc (disc), on which motionless sensory cilia 



B 



cor 



cent 



cor 



sucl; 



FIG. 283. A, Larva of Bugula plumosa ; B, SagittaHection of larva/of Bugula (diagram- 

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

 pyriform organ ; pall, pallial groove ; suck, sucker. (From Korschelt and Heider, after 

 Barrois.) 



appear. In close relation to the ectodermal groove is formed a mass 

 of cells, the pyriform organ (p.) which seems to have a sensory 

 function. 



An alimentary canal is absent in the larva of Bugula when it 

 escapes from the ocecium. After an interval of free existence as a 

 ciliated larva, certain changes appear which lead to a very 

 complete metamorphosis. The sucker becomes everted by a 

 strong contraction of the body, and fixes the larva to some foreign 

 object. The aboral side of the larva becomes greatly extended, so 

 that almost the entire integument of the primary zooid is devel- 

 oped from this part (i.e. from the region occupied by the retractile 

 disc and pallial groove). Accompanying the extension of the 

 aboral surface are the obliteration of the pallial groove and the 

 bending down of the corona towards the oral side. Thus the stage 



