SENSATION. 143 



Tlie lithocysts are almost always freely exposed to the surrounding water upon the margin 

 of tlie umbrella. In Glotssocodon and Car marina, however, they are entirely ineluded within the 

 gelatinous substanee of the umbrella. They are usually sessile, but are occasionally {Ciinina) 

 borne upon a distinct peduncle ; they generally occur in the interval between two marginal 

 tentacles. In Obclia, however, they arc situated on the inner side of the base of a 

 tentacle.' 



The distribution of the litliocysts on the umbrella margin is at tirst always synuuetrical, and 

 their number is equal to or a multiple of that of the radiating canals. It is only in certain cases 

 in which the lithocysts become very numerous, as, for example, in Tima, that this lunnerical law 

 ceases to be apparent. 



We know as little of the function of the lithocyst as we do of that of tiie ocellus. Consisting 

 as it does of a capsule and contained concretions, it presents a structure which has been 

 compared with that of certain organs which are met with in the moUusca and in the Crustacea, 

 and which have in both these groups been generally regarded as organs of hearing, and a similar 

 function has from analogy been attributed to the lithocyst. The analogy, however, is by no 

 means so close as to justify us in attributing an auditory function to the organs in question, 

 while their structure would seem to indicate a relation to light at least as intimate as to 

 sound.- 



nection with the ganglion of the nerve-ring. It is composed of roundish and fusiform cells, and 

 Haeckel regards it as a nervous ganglion. On each side it is prolonged into a flat, longitudinally 

 striated, band-like cord, which is evidently composed of close parallel fibres, and which he considers 

 to be a nerve of sense. These two sense-nerves, after passing up like meridian lines on the walls of the 

 capsule, unite at the opposite pole. Here the fine fibres which compose them appear to become inter- 

 woven into a cord, which, after forming the stalk of the spherical l)ody which immediately encloses the 

 concretions, passes into its interior. Haeckel regards this body as a second internal ganglion. It 

 appears to be a round vesicle, containing within it a mass of small, closely aggregated cells. In the 

 midst of these cells are the concretions, consisting of a calcified organic basis, the organic matter being 

 apparently united with phosphate of lime. He is unable to satisfy himself as to the mode in which 

 the fibres of the sense-nerves terminate within tlie spherical body, though lie believes it probable that 

 their extremities are in connexion with the cells existing in the interior of this body. 



Haeckel's interpretation of the various structures which he thus finds in the interior of the litho- 

 cyst, and regards as ganglia and nerve-cords, may be open to criticism, though it must be admitted 

 that it is not easy to offer any very satisfactory view of them. The two supposed sense-nerves which 

 he has observed running in two opposite meridians on the inner side of the wall of the capsule will 

 suggest a comparison with the meridional striae described above, as apparent on the central pulp of the 

 lithocyst in Campanularia. 



^ In Oceania octona and 0. iurrita, Forbes (' INIonograph') describes a cavity at the base of each 

 tentacle, just below the ocellus, and having within it a vibrating mass. He regards this cavity as a 

 lithocyst, with its contained concretion. A recent opportunity of examining the O. iurrita of Forbes 

 has convinced me that what Forbes took for a lithocyst is only a dilatation of the gastrovascular system, 

 with its contents set in motion by the vibratile cilia of the walls, a suspicion which had already been 

 entertained by Busk and by Gegenbaur. 



" The visual functions of the lithocyst, though maintained by Agassiz, Fr. !Miiller, and partly also 

 by Haeckel, who is inclined to attribute to them a double function of sight and hearing, has been by no 

 one so well supported as by Busk (" On the Anatomy of a Species of Thaumantias," ' Trans. Jlic. Soc. 



