536 DISCOPHORA ; DEVELOPMENT. 



so modified as to produce a more or less complicated hood or 

 covering for the eye. These are called Steganophthalmata, or 

 Covered-eyed Medusce; they are the largest and most highly 

 organised forms of the Order, and their vascular system exhibits 

 a very complicated series of ramifications. In the other section 

 the ocelli, or a part of them, are always placed at the base of the 

 tentacles, and they are never provided with membranous cover- 

 ings ; hence they have received the name of Gymnophthalmata, 

 or Naked-eyed Medusce. They are generally of small size, and 

 their vessels, with very few exceptions, run straight from the 

 centre to the margin of the disk, and never display the same 

 amount of ramification as in the covered-eyed species. In some 

 of them the ovaries are situated in the lower part of the umbrella, 

 in the course of the vessels. 



1190. As already indicated ( 1165), it is in their mode of 

 development that these elegant creatures exhibit most clearly 

 their close relationship to the stationary Hydroid Polypes. With 

 very few exceptions, the eggs of all the Discophora give origin 

 to creatures which in every particular of their structure agree 

 with the characters of the Hydroida, and it is only by a subse- 

 quent process of budding or self-division that the latter again 

 produce the likeness of their free-swimming parents. It was 

 from the observation of these phenomena in the reproduction of 

 the Medusaa that Steenstrup was led to put forward the law of 

 the so-called "Alternation of generations," which has since 

 found applications in so many other groups of animals. In the 

 higher or covered-eyed Medusas, the ciliated embryo (a) re- 

 sembles that produced by the Hydroid Polypes, and like it 

 swims about for a time, and finally attaches itself by one ex- 

 tremity (b). The opposite extremity becomes much widened 

 in proportion to the adherent base, and developes a considerable 

 number of tentacula (c, d), when it closely resembles a Hydrj, 

 and was formerly regarded as a marine form of that genus. At 

 this period it frequently gives birth to young Polypes, in pre- 

 cisely the same way as the Hydra (d). In course of time the 

 body becomes considerably elongate 1, and appirently divided 

 into segments by transverse constrictions (e); these gradually 



