414 Original Articles. [July, 



tical with that of the so-called Medusa. This membrane is a growth 

 from the outer of the two fundamental layers of the body-wall and is 

 contractile, when the zooid is to be locomotive. It constitutes a web 

 uniting the tentacular canals, and thus forming a somewhat campanu- 

 late disc, -while an extension of its margin gives rise to the veil, 

 which partially closes the swimming-bell below. The portion of 

 the body of the polypite in advance of the tentacles is the equivalent 

 of the generative sac of the Gono-zooid. 



We have no intention of troubling our readers with the evidence 

 in detail on which this view rests ; but would merely state, that it is 

 sustained by a careful examination of the development of the Gono- 

 zooid, and by many collateral proofs. The generalization to which 

 we are led will have more popiilar interest. It is this : the reproduc- 

 tive zooid of the Hydroid is in all cases a modified polypite. In its 

 simplest condition it is a polypite without mouth or tentacles ; in its 

 highest, it is a polypite with both, — the tentacles, however, being 

 adapted to the formation of a nutritive and locomotive system, with a 

 view to free existence. The two extremes are connected by a multi- 

 tude of intermediate forms. The so-called Medusa is a sexual poly- 

 pite, with its arms webbed for floating and swimming. 



A very interesting connecting link between the nutritive and sexual 

 bodies in the Hydroid commonwealth is found in the free Gono-zooid 

 of the Clavatella prolifera, Hincks (Plate ii. fig. 7). A representation 

 of the polypite of the Clavatella is given in Plate i. fig. 6. A cluster 

 of the reproductive buds (y, y) is seen pullulating from a point near 

 the base of it. 



The Gono-zooid of Clavatella, though free and locomotive, is not 

 furnished with a swimming-bell — or rather this organ is present only 

 in a latent condition. It is a walker and climber, not a swimmer. 

 It moves by means of sucking-discs attached to the extremity of 

 a branch or fork of the arms (Plate ii. fig. Id). The true ten- 

 tacle terminates in an enlargement (c) which is thickly set with 

 thread-cells. That the membrane which in other species constitutes 

 a swimming-bell exists, we infer from the presence of the radiating 

 canals,* which may be seen passing to the base of the arms 

 (Plate ii. fig. 7). But it is not separated from the body, and is use- 

 less for locomotive purposes. As the zooid walks, the mouth (b) 

 hangs downward, as it does in the swimming forms. At the base of 

 each of the arms is an ocellus or eye-speck (e, e). We recognize, in 

 short, in this form, a Medusiform zooid, without its swimming-bell, but 

 with a compensation for the loss in the shape of suctorial feet. As 

 readily do we recognize in it a polypite, with very slight modifica- 

 tions. Let the reader refer to Plate i. fig. 6, and compare the head 

 of the Clavatella with its free zooid, and he will see at once that very 

 little change is needed to convert the one into the other. Let him 

 suppose the head to be detached at a point nearly opposite to Fig. 6, 

 and the lower portion of the arms to be united by a membrane 



* These were first detected by Krohn. See Wiegmann's 'Archiv.' for 1861, 

 p. 157. 



