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one without mouth and tentacles but specialised for asexual 

 reproduction and called a blastostyle. It is completely 

 enclosed in a vase-shaped extension of the perisarc termed 

 the gonotheca, which has a small opening at the top. 



The blastostyle develops small buds. These are liberated, 

 swim free, and become little medusae; some of them pro- 

 duce ova, others produce sperms. The result of the de- 

 velopment of the egg is a larva called a planula from which 

 a new Obelia colony is developed. 



What is (a) a Larva, (b) a Planula. How does the Planula 

 become a Hydroid ? 



When the young stage of an animal is different from 

 the adult and is free-living it is termed a larva. 



A planula is an oval, ciliated, free-swimming larval 

 form which consists of a layer of ectoderm cells around a 

 central space, which space is subsequently filled more or 

 less completely with endoderm cells. 



The free-swimming planula settles down and attaches 

 itself by the broader end. This end spreads out as a disc 

 from which creeping root-like stolons are budded. The 

 coat of cilia is cast off. The distal end becomes swollen, 

 the body space is enlarged, a mouth is formed, tentacles 

 grow out around it, and the ectoderm secretes a tubular 

 investment of horny perisarc. 



Describe the Medusa of Obelia. 



The medusa and the polyp are homologous. In essen- 

 tials the structure is the same in both. 



The medusa may be regarded as a squat polyp. By 

 flattening out and rounding it has become like a bell with- 

 out a handle but with a clapper. The dome represents 

 the base of the polyp, the clapper represents the oral cone 

 which is a pendant manubrium with a four-sided mouth 

 at the end ; and the tentacles, removed from the oral 

 cone, form the fringe of the dome, 



The manubrium opens into the ccelenteron which is 

 lined with endoderm. Between the endoderm and the 



