INTRODUCTION. XXX111 



tacles. An outgrowth from the margin of the membra- 

 nous bell forms the veil, which partially closes it below. 



The body, containing the stomach, and corresponding 

 with the proboscis or anterior portion of the ordinary 

 polypite, is suspended, as it were, from the centre of the 

 domed roof of the swimming-bell, and hangs free in its 

 cavity. In the alimentary polypite the homologous struc- 

 ture stands erect in the centre of the tentacular wreath. 



The tentacular tubes, which form in the free zooid the ribs 

 on which the umbrella is, as it were, supported, also serve 

 as the canals through which the nutritive fluid circulates*. 

 They communicate, like the tentacles of the polypite, with 

 the cavity of the stomach, and are further united at the 

 margin of the swimming-bell by a circular canal. This ad- 

 ditional structure completes the simple circulatory system. 

 So far it is the only element which has not its homologue 

 or equivalent in the polypite. 



In Clavatella we have an intermediate form, which 

 throws much light on the relation of the medusiform 

 structure to that of the polypite, and very clearly links the 

 two together. In this genus the sexual zooid, though free 

 and locomotive, is not furnished with a swimming-bell. 

 It wants the striking feature of the (so-called) medusan 

 structure, and, instead of floating and swimming, moves by 

 means of suctorial disks, borne at the extremity of a branch 

 or fork of the arms. (Plate XII. fig. 2 a.) But though 

 ambulatory in its habits and destitute of the contractile 

 float, it reminds us at once of the medusa. It has the 

 same general form : as it moves, the mouth hangs down- 

 wards ; and round the body, at the base of the tentacles, 



* In the Hydra, the tentacles are simple tubes into which the fluids pene- 

 trate freelv. 



