1 3S PHYSIOLOGY. 



with great care in certain medusa3 belonging to tlie family of the Geryonida, more especially 

 Glossocodon {Liriope) curyhia and Carmarwn liastata, where it would appear that they are very 

 conspicuous, and well fitted for examination. 



According to Haeckel the nervous system in the Geryonidce consists of a very slender ring- 

 like cord, which runs round the margin of the umbrella immediately below the circular canal, and 

 under each marginal vesicle swells into a ganglion. From these ganglia filaments are sent off, 

 one along the course of eath radial canal as far as the stomach, and one to each of the tentacles, 

 while another penetrates the marginal vesicle in order to undergo within it a peculiar distribution. 

 In the system thus constituted he believes that he has succeeded in demonstrating nerve-elements. 

 The nerve-cells of the nervous ring are contained in the ganglia only ; in the intervening portions 

 the ring presents merely a longitudinally striated appearance. 



While with Haeckel's very detailed description we should hardly be justified in denying 

 the existence of a marginal nerve-cord with ganglia in the Geryonidce, I am by no means 

 prepared to attribute a similar significance to the cord-like structure which may be seen 

 running round the margin of the mnbrella in other medusae (PI. XVI, fig. 8, and woodcuts, 

 figs. 58 and 59). I have in several cases carefully studied this part of the medusa, and have 

 arrived at the conviction that in all these the apparent cord is only the ectodermal layer, which 

 lies immediately upon the distal side of the circular canal, and constitutes the extreme margin 

 of the umbrella, presenting, when viewed along the plane of the codonostome, the appearance of 

 n chord. In the medusa of Campanularia, which Gegenbaur refers to his genus Eacope, and in 

 that of Obelia, which he includes in the same genus,^ I have no doubt as to this being the true 

 interpretation of the supposed nerve-cord, while the structures assumed to be ganglia are only 

 thickenings of the ectoderm at the points which give attachment to the litltocyds or sense- 

 capsules, to be presently described. 



Besides the structures to which the significance of a nervous system has been thus 

 assigned, certain bodies which have been long known and regarded by common consent as organs 

 of sense, hold a prominent place in many hydroids. They occur only in the meduste, and are of 

 two kinds — the ocellus and the lUhocyst. 



The Ocellus. — The ocellus consists of a little mass of pigment, forming a well-defined 

 coloured spot, in some species black, in others vermilion or deep carmine. In most cases (PI. V, 

 fig. 3, PI. VI, fig. 3, and PI. IX, fig. 8) no other structure can be detected in the ocellus ; 

 l)ut sometimes (PI. XVII, fig. 5, and PI. XVIII, fig. 6) a transparent, refracting body may 

 be seen immersed in the pigment mass. The ocellus is always situated in the walls of the 

 bulbous dilatation which exists at the root of the marginal tentacle of the medusa (woodcut, 

 fig. 50), where it lies very superficially, being imbedded exclusively in the ectoderm, while a 



' Tiie genus Eucope was founded by Gegenbaur for certain small medusse ■which are known to be 

 the planoblasts of campauularian hydroids. He subdivides Lis genus into a deep-belled and a shallow- 

 belled section — sections, however, which differ from one another by characters which are at least of 

 generic value. The deep-belled forms are the planoblasts of the true Campamdaria, while those with 

 shallow bells are the planoblasts of another campanularian genus {Laomedea of authors, in part), and 

 liad already been described by Peron and Lesueur, under the generic name of Obelia. Eucope, as a 

 generic appellation, must therefore be suppressed in favour of the older names of Campamdaria and 

 OheUa, by which the deep-bcUed and shallow-belled forms must be respectively designated. 



