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ectoderm, which forms the outer surface of the medusa, 

 is an extensive mesogloea. The greater bulk of the medusa 

 consists of mesoglcea jelly. 



The ccelenteron gives off four radial canals, channels of 

 endoderm, which run at right angles through the mesogloaa 

 and open into the circular canal of the margin. By means 

 of these canals food is distributed throughout. 



The tentacles have ocelli ("eyes") at their bases; and 

 eight of them (two in each quadrant) have also " otocysts." 

 These are sense organs by which the medusa changes or 

 corrects its position in the water. Each is a sac lined 

 by ectoderm cells, some of which develop fine sensing 

 hairs, while others secrete the limy concretions in the sac. 



There are four gonads (reproductive organs) on the 

 underside of the medusa, one below each radial canal. 



The ectoderm cells of the margin and underside have 

 contractile processes, and by this means the medusa swims, 

 by rhythmic contractions of the bell. 







What is a Velum or Craspedon? 



A narrow fold of the margin of the medusa bell produced 

 inwards like a shelf. Medusae with a velum are described 

 as craspedote, those without a velum as acraspedote. 



Give the Meaning of the^Terms Leptomedwce and Antho- 

 meduscB. 



Medusae which have " otocysts," and which have gonads 

 in connection with the radial canals, are termed Lepto- 

 medusse (e.g., Obelid). 



Medusas which are without " otocysts," and which have 

 the gonads on the manubrium, are called Anthomedusse. 



In what ways does Tubularia differ from Obelia ? 



It is a gymnoblastic hydroid, i.e., one which has no cup- 

 like extension of perisarc around the polyp. It has two 

 sets of tentacles, long ones round the oral cone and short 

 ones encircling the mouth. The medusae are not set free, 

 they remain attached as gonophores t 



