546 



EVOLUTION OF ANIMALS 



TENTACLES 



Part V 



CIRCULAR 

 CANAL 



MOUTH 



NERVE 

 GANGLION 



STOMACH 



RETRACTOR MUSCLE 

 INFUNDIBULUM 



SHELL 



OR 



ZOOECIUM 



Fig. 27.12. Structure of one individual greatly enlarged of a bryozoan colony 

 such as the common marine Bugula. (Courtesy, Miner: Fieldbook of Seashore 

 Life. New York, G. P. Putnam's Sons, 1950.) 



They are shaped like birds' heads, whence they are called avicularia. Under 

 a lens they can be seen snapping their bills with every vibration in the 

 water, and if any particle touches them they snap shut in a viselike hold. 

 They catch and kill the microscopic organisms that continually settle on the 

 bodies of the bryozoans and constitute private cleaning squads. Starfishes and 

 sea urchins have similar mechanisms, but they are unknown in higher animals. 

 No dog has pincers to trap his fleas. 



Movements. A lophophore with all its tentacles can be instantly jerked out 

 of sight by the bands of muscle in the body cavity. But, its emergence is slow 

 and the tentacles spread forth seemingly with great caution, actually because 

 each one is expanded by fluid flowing slowly into it (Fig. 27.13). 



Reproduction. Bryozoans are hermaphroditic and ovaries and testes develop 

 in the coelom in which the eggs are fertilized. The embryo develops in a brood 

 pouch that opens out of the coelom (Fig. 27.12). In the marine species, the 

 ciliated trochophore swims about freely for a short time, then becomes attached 

 to seaweed or rock (Fig. 27.1). 



