142 



PHYSIOLOGY. 



In most cases no del'uiitc structure can be detected in the walls of the external capsule. In 



an nndescrihed species of Tynropsis, how- 



FiG. 59. 



Medusa of Obelia geniculata. 



A. Part of the umbrella-margin, a, a, marginal tentacles; b, h 

 thickened ectoderm at the base of the marginal tentacles; c, c, large 

 cell forming the proximal extremity of tlie marginal tentacle where it 

 is plunged into the gelatinous substance of the umbrella; d, d, d, ecto- 

 derm forming the extreme margin of tlie umbrella; e, circular canal; 

 /,/, gelatinous substance of the umbrella ; g, lithocyst. 



B. Meridional section of umbrella-margin, a, marginal tentacle; 

 A, thickened ectoderm at base of marginal tentacle; c, large cell form- 

 ing the proximal extremity of marginal tentacle; d, ectoderm of ex- 

 treme margin of umbrella ; e, circular canal in transverse section ; /, 

 gelatinous substance of umbrella ; (j, lithocyst. 



C. Structure of epithelium clothing the inner surface of the umbrella. 

 In feveral of the cells the nuclei have undergone division. 



ever (Tj/aropsis scotica, woodcut, fig. 57), 

 these walls are composed of very distinct 

 prismatic cells, lined with a finely granular 

 layer, which immediately invests the pulp 

 and tlie zone of concretions imbedded in its 

 surface, and I have noticed a similar struc- 

 ture in another medusa allied to Tliaumantias. 

 Gegenbaiir has pointed out the existence of 

 a layer of polygonal cells, which lines the 

 capsule in one of the Geri/on'uhe, an observa- 

 tion which has been confirmed by Haeckel in 

 other members of this family ; and in the 

 medusae of Campanularia and Obelia de- 

 scribed above there is indication of a special 

 lining of the capsule. 



Hensen, in his remarkable memoir on 

 the auditory organ of the Decapod Crusta- 

 cea, describes the lithocyst of a medusa, 

 which he refers to Gegenbaur's genus 

 Eitcope, and maintains that the concretion 

 is connected to the walls of the capsule by 

 a bundle of very fine hairs (" auditory 

 hairs").' This observation of Hensen, how- 

 ever, has not been confirmed by other in- 

 vestigators, and I consider it almost certain 

 that the meridional strife described above 

 have given rise to the appearance which 

 Hensen has mistaken for hairs.^ 



' " Studien iiber das Gcliiirorgan der Decapoden," ' Zeit. fur Wissensch. Zool.,' vol. ^\\\, p. 355, 

 note. 



- The lithocyst has been studied with great care by Haeckel (op. cit.) in Glossocodon eurybia and 

 Carmarina hasiata, Geryonidan Medusse which, from their large size, offer special facilities to the 

 observer, though they belong to a peculiar type, and their litliocysts, as described by Haeckel, present 

 features which differ in many points from the structure which, as we have already seen, these bodies 

 present in Campanularia and Obelia. 



In the Medusaj which form the subject of Haeckel's observations the lithocyst is completely included 

 within the gelatinous substance of the umbrella-margin. It is described as a transparent sphere seated 

 on the upper side of a ganglion of the nerve-ring. Its walls are formed of a homogeneous membrane lined 

 with a pavement epithelium, and it is filled with what appears to be a watery fluid, into which projects 

 from the upper free wall of the capsule a clear spherical body, attached to the wall by means of a broad 

 short stalli, and containing within it one or more small, concentrically laminated concretions. 



Withiu the capsule, on its base, there is seated a cushion-like body, apparently in immediate con- 



