70 



IXVERTEBKATE ANIMALS, 



As ivg;ird.s tlie stnn-tnrc of svich Jfeihtsa', there ii nothing to 

 enable the ol)ser\'er tu kiifiw foi' certain whether a given Jelly-fish 

 is merely the free generative liiid 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 hydrosnma is free and oceanic, the organism living 

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

 tnmsparent, gelatinous disc (.ir swimming -bell (the nectocalyx) 

 1)3' tlie jiulsations of which the animal i.s driven through the water. 

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



Fig. 3S. — Tmchyneraa digitate, a naked- eyed Mfclnsa, female, enlarged. (After A. 

 Agassiz.) p Manutriuin or central ixilypite; (One of the tentacles ; c One of the 

 ga.stro-vasenlar canala ; o One of tin; ov;nies. 



:i iiingle polyjjite (the manubrium), which bears to the disc the same 

 r:'; ;,tive position as the cla|i|icr does to an ordinary hand-bell. The 

 di-ital end of the central ]...lypite (fig. 3S) is furnished with a 

 ninuth, tlie lips of which are dflen jin. longed into four longer or 

 shoi-ter lobes or jirncesses. 'I'lie UK.iilh opens into a digestive sr,c, 

 oc.npying the .-ixis (jf the iinlypite ; and from the njiper end of this 

 ]>r.i.eed four railiating c.-uials, which run in the sub.stauce of the disc 

 to lis margin, where they arc uiiiti'd by a, single circular vessel, 



