XXX11 INTRODUCTION. 



variation on that which we find in the polypite, exhibiting 

 the same principal elements, which are modified in con- 

 formity with the new conditions of being. The free gono- 

 zooid is essentially a polypite suspended in a contractile 

 bell, which bears it through the water. We have only to 

 imagine an ordinary alimentary zooid, detached and with 

 its tentacles united by a web for a portion of their length, 

 to have a structure closely resembling the (so-called) 

 medusa. In Campanulina acuminata the arms of the poly- 

 pite are, to some extent, palmate (Plate XXXVII. fig. b], 

 and in the delicate web which connects them we have the 

 homologue of the swimming-bell*. A polypite of this 

 genus, separated from its colony, and floating by means of 

 its tentacular disk, would suggest at once the aspect and 

 habit of the medusiform zooid. 



In its highest form the sexual polypite takes on a struc- 

 ture which fits it for independent existence. The tubular 

 appendages, which in the nutritive zooid are mere prehen- 

 sile organs f, are now connected for the greater portion of 

 their length by a highly contractile membrane, and form 

 a bell or disk, which serves as a float and a propeller. The 

 extremities remain free, and discharge the office of ten- 



* Prof. Allman has remarked that in all cases the tentacles of the poly- 

 pite are necessarily included in the thickness of the body-walls for some dis- 

 tance from their origin. And this included portion he regards as the repre- 

 sentative of the radiating canals. (" Report on the Reproductive System in 

 the Hydroida," Rep. Brit. Assoc. for 1863, p. 364.) 



f The view which regards the tentacles of the polypite and the radiating 

 canals of the (so-called) medusa as homologous parts is confirmed by many 

 considerations. It is worthy of remark that in some cases (e. g. Zygodacfyla) 

 the canals increase in number as the zooid advances towards maturity, just 

 as the tentacles of the polypite do ; and the course of development is the 

 same in both. The new canals originate at (lie base of the manubrium, and 

 irradually grow downwards to the circular vessel on the margin. 



