SENSATION. 



Ml 



they frequently appear with great distinctness, more especially under the action of dilute acetic 

 acid.' 



Fig. 58. 



Part of the Umbrella-margin in the Medusa of Campanularia Johnatoni. 



a, cavity of circular canal; b, cavity of a radiating canal; c, c, e, c, endodermal lining of 

 radiating and circular canals ; rf, rf, d, d, ectoderm forming tlie extreme margin of the umbrella ; 

 e, ectoderm of marginal tentacle ; /, thickened ectoderm at the base of the tentacle loaded with 

 thread-cells and directly continuous with d, the ectoderm of the umbrella-margin ; g, marginal 

 ectoderm thickened where it lies over the spot from which a new tentacle is to spring; 

 ht lithocyst. 



In Obclia genicidcda, the litbocysts are also eight in number, two being situated in each of the 

 four interradial spaces (woodcut, fig. 59 Kg, and By). They are sessile on the cord-like margin 

 of the umbrella.and are placed each at the subumbrellar side of the base of a tentacle, but not 

 exactly in its meridian plane. The structure of the lithocyst resembles that just described in 

 CampamiJaria Johnsfoni, consisting, like it, of a spherical capsule with a contained pulp, which 

 nearly fills it, and which is excavated at the distal pole of the capsule into a cavity in which 

 is the spherical refracting concretion. I could, however, obtain no distinct evidence of the meri- 

 dional striae visible in the lithocvst of C. Johidoni? 



' Busk appears to have noticed these striffi, for in a paper which he read before tlie Microscopical 

 Society of London (' Trans. Mic. Soc. Lond./ vol. iii, p. 22), and which gives the most complete 

 account of the lithocyst we had possessed up to the time of its publication, the author describes " au 

 indistinctly fibrous, radiating appearance," which might sometimes be detected in the more external 

 parts of the contents of the lithocyst iu a Thaumantias-\^& medusa. 



- Keferstein and Ehlers ('Zoolog. Beitrage,' p. 88, pi. xiii, fig. 4) describe the Eucope polystijla 

 of Gegenbaur, which is, however, really an Obelia. They give a good figure of the lithocyst, but they 

 mistake the cavity in which the concretion is contained for a peduncle projecting into the capsule from 

 its distal wall, and carrying the concretion on its summit. A similar mistake seems also to have been 

 made by Fr. Miiller (" Ueber die Randblaschen der Hydroidenquallen," in Max Schultze's ' Arch. f. 

 Mic. Anat.,' 1865, vol. i, p. 143), in his account of the lithocyst of an allied Medusa. 



