JELLY-FISHES. 95 



The resemblances between Lucernaria and Periphylla are for the most jiart 

 anatomical in character, and so little is known of the development of both that there 

 is little possibility of a comparison in this particular. The comparison, step by step, 

 of the many likenesses between the two genera would take iis too far into special 

 studies of the peculiar anatomy of them both, but these points of likeness are of a most 

 important character, and show that, notwithstanding one form is attached and the 

 other free, they may be closely allied to each other. 



The character of the development, and the different larval conditions which the 

 Discophora pass through in that growth, present some of the most interesting facts in 

 regard to these animals. In the progress of research into the anatomy and classifi- 

 cation of the lower forms of animals three curious zoophytes, placed in three genera, 

 had been described by different naturalists. These genera were called Scyphistoma or 

 Scyphostoma, Strobila, and Ephyra. It was suspected that they were not adults, but 

 in the early days of the history of marine zoology no one had any idea that these 

 animals had close relationships with one another. The first and most important step 

 in a true understanding of the nature of the larviB of the medusse was made by Michael 

 Sars, by whom it was found that these three genera were one and the same, and 

 Steenstrup, shortly after, recognized that there exists in the medusas a true alterna- 

 tion of development such as the poet Chamisso had pointed out is found in the forms 

 of the Ascidian genus Salpa, known as the " chain form," and the solitary or asexual 

 individual. 



In late summer and autumn specimens of Cyanea of large size are often taken in 

 which the membranous fold which hangs downward from the oral region of the disk 

 is loaded with white packets or bundles. These bundles are composed of ova, and if 

 they are examined with a microscope of even low magnifying power will be found to 

 have already entered upon the first steps in their development. In other words the 

 genus Cyanea carries its young about and protects them in the folds of the mouth, 

 from the very youngest to some of the higher larval conditions. The highest con- 

 dition which it has in its career in the mouth-folds of the parent is what is known as a 

 plaiiula. The planula is an elongated, spheroidal body whose walls are formed of two 

 or perhaps three layers, within which is a small cavity, and whose outer surface is 

 covered with vibratile cilia. The function of the vibratile cilia is that of progression 

 through the water, and, as a consequence, immediately on attaining this condition it 

 swims away from the fostering care of the parent, and shifts for itself in the water. 

 In this free-swimming or planula stage it remains until, freighted by the weight of 

 increased age, it can no longer swim through the water by the ciliary movements. 

 When that age comes in the progressive growth of the Cyanea, the embryo, which 

 was formerly spheroidal in shape with symmetrical poles, becomes pear-shaped, 

 presenting an obtuse and a pointed pole which can easily be distinguished. The larva 

 next attaches itself by one of these poles to some fixed object, and the two following 

 stages in its growth are passed through in that condition. 



Immediately after attachment there forms at the free end of the body a circle of 

 little protuberances which, as the growth goes on, become more and more elongated, 

 while in the centre of the circle, in the periphery of which they lie, an opening is found 

 leading into a cavity in the interior of the body. The resemblance of the young 

 animal, in this first of the attached forms, to the common fresh-water ^(?ra, which has 

 been described, is very striking. The larva was one of those three supposed genera 

 mentioned above which were foi-merly thought to be widely different from any of the 



