LECTURES ON EMBRYOLOGY. 



[PLATE XXVII- 



enough to be seen in the living animal, under a 

 power of a few hundred diameters. 



Beside this, there is around the upper part of 

 the alimentary tube, a linear circle of another sub- 

 stance, from'which radiate four threads, following 

 the direction of the alimentary tubes, and ex- 

 tending towards the periphery, which reach there 

 the spherical, colored bodies, now generally con- 

 sidered as eye specks, and uniting with each other, 

 form a circular thread all around the margin of 

 the disc. This apparatus I consider to be the 

 nervous system. Its position is the same as in the 

 other radiated animals, a circle around the alimen- 

 tary tube, with diverging rays, ending in the small 

 colored organs which since discovery (Plate XIX, 

 fig. M) have been considered as Ehrenberg's eye 

 specks, similar to those which I have already no- 

 ticed, at the end of the rays of star-fishes, and 

 upon the plates of Echinoderms. The fact of these 

 threads going to those spots (Plate XXVII, fig. C), 

 leaves no doubt, in my mind, that it is a complete 

 radiating, nervous system, similar to that of star- 

 fisnes. So that the structure of medusae, though 

 peculiar in itself, by the remarkable mode of dis- 

 tribution of its inner cavity, which does not con- 

 stitute an alimentary canal proper, resembles al- 

 most entirely the structure of Echinoderms, and 

 constitutes one of the main classes among Radiata 

 as Echinoderms do. The position of these ani- 

 mals was mentioned. They swim free, the mouth 

 downwards, the sphere upwards ; and this is al 



ways the position which the Echinoderms assume, 

 The Echini, Sea Urchins, walk about, the mouth 

 downwards. Star-fishes walk about, the mouth 

 downwards. The Crinoids, however, stand up- 

 right, the mouth upwards, and this is the position 

 which the animals of the lowest class assume. 



In all Polypi, the main body stands upon a stem, 

 the mouth upwards ; and we have also among Me- 

 dusae [Plate XIX, figs. G & I] a similar condition 

 during one period of growth. 



When the young animals are fixed by the lower 

 portion of their body, the tentacles, or appendages, 

 which every where hang downwards, stand here 

 upwards; so that you see how remarkably the lower 

 types among Echinoderms resemble in this respect 

 the Polypi in their constant position, and how in 

 youth, Medusae, in that respect also agree with 

 Polypi. There is a constant recurrence of charac- 

 ters from one of these classes to another. They are 

 interwoven in a most remarkable manner. 



All Jelly-fishes are generally considered as simple 

 animals; but I am satisfied that there are, on the 

 contrary, highly complicated ones among them. 

 The Physophora differ indeed widely from the 

 other Medusa, by their diversified appendages, as 

 is shown by the structures figured on this diagram. 

 [Plates XXIII and XXVI J I am prepared to show 

 that these are compound animals, composed of 

 groups of individuals of different kinds ; indeed, 

 compound animals as we find them among Polypi. 

 [PLATE XXVIII HYDRA-CAMPANULART A ] 



In order to show that this is the case, let me il 

 lustrate in detail the metamorphoses of Medusae. 

 Let me also refer you to some Polypi, [Plate 

 XXVIII] in which you see how individuals are 

 combined together, forming a compound stick. 

 Though all these individuals are of different ages 

 and have been found successively, they form living 

 colonies, as it were, of successive generations, uni- 

 ted by material connections, -which remain for life 

 the new individuals not separating during life. 

 In others, the successive buds may be more or less 

 different, and nevertheless remain united in one 

 common colony, or as it %vere form a community 

 of individuals closely united, though differing in 

 age, size, form, and even in sexes. Such is the 

 case at least in the Campanularia, figured Plate 

 XXVIII. But there are also among Polypi simple 

 ones, like this little Hydra, I Plate XXIX]. 



When alive the Medusa lays eggs, and the em- 

 bryos are hatched, these germs swim freely, and 

 then become attached. And the point by which 

 they become attached grows longer [PL XIX, B], in 

 proportion as the mass above grows larger. The 



