DIVISIONS OF THE HYDROZOA. 69 



special organs, which are developed in the course of the radiating 

 canals of the disc. The resulting embryos are minute free-swimming 

 bodies, covered with cilia, which finally fix themselves, and develop 

 into the plant-like colonies from which the medusoids were derived. 



Order IV. Medusid*. 



Among the most familiar and beautiful of all marine organisms 

 are the delicate, transparent, bell-shaped creatures generally known 

 as Jelly-fishes. All the organisms, which are generally spoken of 

 as Jelly-fishes, belong to the Hydrozoa, but they differ greatly as 

 to their precise nature. The great gelatinous discs which are so 

 frequently cast up on the sea-shore, and which are more especially 

 spoken of in common language as "Jelly-fishes," are mostly the 

 reproductive zooids of a particular group of Hydrozoa (the Lucer- 

 narida), and will be subsequently noticed. Less familiar than 

 these, but equally abundant, are certain, usually much smaller, 

 Jelly-fishes which are found floating in the sea near the surface, 

 often in vast numbers, at particular seasons of the year. Many of 



Fig. 37. — a A meclusiforin gnuopliore iTliaumantiaB) seen in profile, showing the 

 central polypite, the radiating and circular canals of the disc, the reproductive 

 organs borne in the course of the former, the marginal bodies and tentacles ; & The 

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



these comparatively minute Jelly-fishes are in reality, as previously 

 noted, neither more nor less than the free-swimming reproductive 

 buds of the Hydroid Zoophytes, or of other groups of Hydrozoa. 

 That this is their true nature is shown by the fact that the eggs 

 which they produce develop themselves not into fresh Medusce, 

 but into various other forms of Hydrozoa, which may be either fixed 

 or oceanic. On the other hand, there are certain Jelly-fishes (Trachy- 

 nemidce, ^c/hiidce, &o.) the eggs of which really give origin to fresh 

 Jelly-fishes. These, therefore, must be considered as a special group 

 of the Hydrozoa, to which the name of Medusidce may be applied. 



