128 MANUAL OF ZOOLOGY. 



the order is very much restricted, 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 Gym- 

 nophthalmate Medusa of Professor E. Forbes, the Steganoph- 

 thalmate Medusa of the same author being now placed in the 

 sub-class Lucernarida. 



The hydrosoma of one of the Medusida ( = a Gymnophthal- 

 mate 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 (figs. 52, 53). The 



Fig. 52. * Morphology of Medusidae. a A Medusoid (Tkaumantias) seen in profile, 

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

 marginal vesicles and tentacles, and the reproductive organs ; b The same viewed 

 from below. The dotted line indicates the margin of the velum. 



interior 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 pro- 

 duced 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 distin- 

 guished from the somewhat similar "umbrella" of the Liccer- 

 narida. The endodermal lining of the central polypite or 

 " manubrium " (sometimes called the " proboscis ") is pro- 

 longed into four, sometimes more, radiating canals, which run 

 to the periphery of the nectocalyx, where they are connected 

 by a circular canal which runs round its circumference, the 

 whole constituting the system of the " nectocalycine canals " 

 (often called the " chylaqueous " or " gastro-vascular canals"). 

 From the circumference of the nectocalyx depend marginal 



* The form here figured (fig. 52), though in all respects anatomically 

 identical with the true Medusa, and originally described as such, is now 

 known to be in reality the medusoid bud of a fixed Hydroid. It illustrates 

 the structure and form of the Medusa, however, just as well as though it 

 were completely independent in its development. 



