GENERAL ZOOLOGY 



Brood pouch 



Zygote 



Lophophore ■ 



r^ Avicularium 



Caecum 



Rectum 



(y^p^ Pharynx 



Central 

 stomach 



Fig. 12.11. Bus,ula: structure and reproduction. A, transfer of a zygote from the coelomic 

 cavity of a zooid into the brood pouch, where it will develop into a cihated larva. B, in- 

 dividual retracted and at rest; note the position of the lophophore within the vestibule. (Re- 

 drawn, after Gerwerghagen, from C. Cori in W. Kiikenthal and T. Krumbach, 1938, Handbuch 

 der ^oolog^ie.) 



extending from the end of the caecum to the posterior body wall. This 

 mesentery usually bears the testis, and an ovary develops in the parietal 

 peritoneum of the same individual. Fertilization of a single ovum occurs 

 within the coelom, and the zygote is passed forward into a brood pouch 

 which develops like a hood extending distally over the lophophore (Fig. 

 12.11). Within this pouch the embryo matures, becoming a ciliated larva 

 which escapes for a brief period of free existence. With the release of this 

 larva, a second egg matures in the ovary, is fertilized, and takes its place 

 in the brood pouch. The free-swimming larva (Fig. 12.12) settles upon the 

 substratum, and its ensuing development into a mature animal involves a 

 radical metamorphosis. In this transformation practically all signs of organ- 

 ization are lost, the organs of the future adult arising from an apparently 



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