318 



ZOOLOGY 



SECT. 



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

 cor.) is formed round the equator of the embryo and becomes 

 provided with cilia. A sac-like, afterwards beaker-shaped, invagi- 

 nation of the ectoderm on what is destined to become the oral 

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

 (Fig. 257 suck) which afterwards serves to fix the larva. A second 

 imagination of the ectoderm in the region of the corona forms 

 the cctodermal 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 appear. A glandular organ 

 is developed by ingrowth of the cells lining the ectodermal groove. 

 An alimentary canal is wanting in the embryo when it escapes 

 from the ocecium, but develops at a later stage. The sucker 



disc 



B 



cor 



cent. 



cor 



suck 



FIG. 2">7. A Larva of Buguia plumosa ; B. Sagittal section of larva of Bntrula (diagram- 

 matic), cent, parenchyma ; cur, corona; <'<>, retractile disc; pall, ectodermal groove; suck, 

 sucker. (From Korschelt and Heider.) 



becomes everted by a strong contraction of the body, and fixes the 

 larva to some foreign body. The aboral side of the larva becomes 

 greatly extended, so that almost the entire integument of the 

 primary zooid is developed from this part. The retractile disc, 

 ectodermal groove, and glandular organ are now withdrawn* into 

 the interior. The corona and glandular organ disappear; the 

 retractile disc takes part in the development of the organs of the 

 primary zooid. A sac, the wall of which is composed of two layers, 

 now becomes formed, and gives rise to the organs of the adult 

 zooid : the inner layer of this sac is formed by imagination from 

 the retractile disc, while the outer layer appears to be formed from 

 the central mass of tissue, the remainder of which partly goes 

 to form the parenchyma of the adult, but partly contributes to 

 the formation of a rounded granular mass the brown body in 



