84 



What is the Interest of Hydractinia ? 



It is hydra-like and forms unbranched colonies, often 

 upon the whelk shells tenanted by hermit-crabs (commen- 

 salism). 



These colonies exhibit polymorphism and division of 

 labour. Some of the polyps are ordinary nutritive in- 

 dividuals, some (the tactile palpons) are longer and without 

 mouths, others with rudimentary tentacles and with or 

 without mouths are specialised for reproduction ; and a 

 number are abortive " persons," functioning as the pro- 

 tective spines of the colony. 



Aurelia (The common Jelly-fish). 



W hat are the chief Structural Features of the common Jelly- 

 fish? 



It is acraspedote ; there is no velum. The mouth has 

 four long and frilled lips. The ccelenteron is a wide four- 

 lobed space, and on the floor of the lobes are 'the four 

 semicircular gonads which are developed from the endo- 

 derm. Each gonad has, on its inner side, a row of gastral 

 filaments. There are eight straight radial canals and 

 eight which branch. There is no nerve ring, and the eight 

 sense organs are tentaculocysts or rhopalia. 



How does Aurelia reproduce ? 



The method of reproduction is sexual. Aurelia is dioe- 

 cious, i.e., the sexes are separate. 



Write a short Account of the Life-history of Aurelia. 



In early summer the developing embryos are carried 

 about in pockets of the manubrium. Later they go free. 

 The free-swimming planula soon attaches itself to a frond 

 of seaweed and becomes a little hydra-like polyp called 

 a hydra-tuba. Others, like itself, arise as. buds from its 

 creeping stolon. 



In late autumn the hydra-tuba loses its tentacles, the 

 edge of the oral disc becomes lobed, and an oral cone is 

 developed. It elongates and annular constrict ions . 



