70 INVERTEBRATE ANIMALS. 



As regards the structure of such Medusce, there is nothing to 

 enable the observer to know for certain whether a given Jelly-fish 

 is merely the free generative bud of some fixed Hydroid, or whether 

 it is an independent organism. In either case, the anatomical 

 structure is essentially the same, and may be briefly stated as 

 follows : The hydrosoma is free and oceanic, the organism living 

 near the surface of the sea. The body is composed of a thick, 

 transparent, gelatinous disc or swimming - bell (the nectocalyx) 

 by the pulsations of which the animal is driven through the water. 

 From the under surface or roof of this bell-shaped disc is suspended 



Fig. 3S. — Trachj/nenm digitale, a nalted-eyed Medu.sa, female, enlarged. (After A. 

 Agassiz.) 3? Manubrium or central polypite ; ( One of the tentacles; c One of the 

 gastro-vascular canals ; o One of the ovaries. 



a single polypite (the manubrium), which bears to the disc the same 

 relative position as the clapper does to an ordinary hand-bell. The 

 distal end of the central polypite (fig. 38) is furnished with a 

 mouth, the lips of which are often prolonged into four longer or 

 shorter lobes or processes. The mouth opens into a digestive sac, 

 occujjying the axis of the polypite ; and from the upper end of this 

 proceed four radiating canals, which run in the substance of the disc 

 to its margin, where they are united by a single circular vessel. 



