CCELENTERATA : HYDKOZOA. 85 



The Medusidce comprise most of the organisms commonly 

 known as Jelly-fishes or Sea-nettles, the last name being de- 

 rived from the property which many of them possess of 

 severely stinging the hand, this power being due to the pre- 

 sence of numerous thread- cells. As employed by ^modern 

 naturalists, the order is very much restricted, and it is by no 

 means improbable that it will ultimately be entirely done 

 away with, very many of its members having been shown to 

 be really the free generative buds of other Hydrozoa. As 

 used here, it corresponds to part of the GymnophthalmoJe 

 Medusce of Professor E. Forbes, the Steganophthalmate Medusa 

 of the same author being now placed in the sub-class Lucerna- 

 rida. 



The hydrosoma of one of the Discopliora (=aGymnoph- 

 thalmate Medusa) is composed of a single gelatinous bell- 

 shaped swimming organ, the 'nectocalyx' or ' disc,' from the 

 roof of which a single polypite is suspended (Jig. 20). The in- 



a 



Fig. 20. Morphology of Medusidse. a. A medusid (TJiaumantias) seen in profile, 

 showing the central polypite, the radiating and circular gonocalycine canals, 

 the marginal vesicles and tentacles, and the reproductive organs. 6. The 

 same viewed from below. The dotted line indicates the margin of the 

 velum. 



terior of the nectocalyx is often called the 'nectosac,' and the 

 term ' codonostoma ' has been proposed to designate the open 

 mouth of the bell. The margin of the nectocalyx is produced 

 inwards to form a species of shelf, running round the margin 

 of the mouth of the bell, and termed the ' veil ' or ' velum,' 

 by the presence of which the nectocalyx is distinguished from 

 the somewhat similar 'umbrella' of the Lucernarida. The 

 endodermal lining of the central polypite or ' manubrium ' 

 (sometimes called the ' proboscis) is prolonged into four 

 radiating canals, which run to the periphery of the necto- 

 calyx, where they are connected by a circular canal which 

 runs round its circumference, the whole constituting the 

 system of the ' nectocalycine canals ' (formerly called the 

 ' chylaqueous canals'). From the circumference of the necto- 



