XXXIV INTRODUCTION. 



are a number of coloured ocelli. On examination it is 

 found to be furnished with radiating canals, which are 

 short and broad, and correspond in position with the arms. 

 There is also a circular canal. 



The tentacles exactly resemble those of the polypite, 

 with the exception of the branches bearing the adhesive 

 disks. In other respects there is the closest resemblance 

 between the free zooids of Clavatella and its polypite. 

 The oral extremity of the latter, detached from the long 

 pedunculated body which supports it *, requires very slight 

 modification to convert it into the curious ambulatory 

 structure which is charged with the propagation of the 

 species. Putting aside for the moment the eye-specks and 

 the locomotive appendages which are superadded to the 

 stationary organism, there is but a single point of diffe- 

 rence between the two of any significance. In the gono- 

 zooid a larger portion of the tentacles is included in the 

 body-walls than in the polypite, and the included portions 

 are united by a circular vessel f. The sexual zooid of 

 Clavatclla may be regarded as a partially developed me- 

 dusa ; it is as clearly a slightly modified polypite. If we 

 imagine the extension of the body- wall upon the tentacles 

 to be carried somewhat further, we have the perfect 

 swimming-bell. 



The close resemblance between the gonozooid and the 



* This seems to have its homologue in the pedicle by which the gonozooid 

 is attached to the parent stock, a portion of which it bears with it for a time 

 after liberation, 



It may be noted further that the reproductive buds of Chivatdla are de- 

 stitute of any ectothecal covering, and exactly resemble the polypites in their 

 mode of growth. 



t The study of Cluvah'Ua leaves no room for doubt as to the homological 

 relation between the mdiating canal and the tentacle 



