Invertebrate Gallery of the Indian Museum. ig 



and into cavities in this the zooids can withdraw them- 

 selves. All these gradations may be well studied in the 

 specimens and models exhibited in the collection. (Cases 

 3-4). 



Another complication found in these compound colonies 

 is due to the fact that a physiological division of labour 

 takes place among the zooids of which the individual 

 colonies are composed, some of the zooids remaining 

 devoted entirely to nutritive functions and retaining the 

 hydroid form, while others become specialized for repro- 

 duction, and undergo much modification in form. The 

 essence of this modification is that the reproductive zooids 

 (or "gonozooids") lose their fixed attachment and become 

 free-swimmers. The explanation of their becoming free- 

 swimmers is that the eggs and young which they produce 

 may be widely scattered, and so may be more likely to find 

 food and to live and grow than they would be if they fell 

 close to the already well-established parent colony. 



The reproductive polyps are known as Medusse .- they 

 may be regarded as hydra-ioxm polyps, which have become 

 greatly flattened out at the aboral pole into a disk, and 

 then bent round into the form of a bell, from the centre 

 of which the oral end of the polyp (the mouth) hangs 

 down like a hollow clapper (" manubrium " or " gastric 

 peduncle "). 



At the same time the mesoderm in the wall of the bell 

 becomes much thickened, and the gastral cavity in the 

 wall of the bell becomes obliterated, except at the cir- 

 cumference of the bell, where it remains as a canal ("cir- 

 cumferential canal"), and along certain definite lines 

 (usually four in number) which lead from the cavity of the 

 clapper or gastral peduncle to the circumferential canal. 

 These canals are known as " radial canals", and they 

 carry digested nutriment to all parts of the bell. They are 

 well seen in the spirit-specimen of Olindias mulleri, 

 in Case 3, 



Further, the inside edge of the bell becomes thickened 



B2 '-^ 



I 



