38 HTDROZOA. 



containing one or more solid, motionless concre- 

 tions, immersed in a transparent fluid. These 

 concretions are oval or spherical in form, and 

 appear to be composed of carbonate of lime. 

 From the inner wall of the comparatively large 

 vesicle in Gei^onia arises a short stalk, which 

 expands to form a delicate membrane around the 

 solitary concretion. In some forms, a much 

 thicker covering invests, along with the concre- 

 tion and a number of minute molecules, a round 

 or oval body, not unlike an endoplast. Many 

 other modifications of the vesicles have been de- 

 scribed. 



The pigment-spots, otherwise termed " ocelli " 

 and "eye-specks," consist of aggregations of colour- 

 ing matter, enclosed in distinct cavities. The 

 tint of these bodies is often extremely brilliant, 

 shades of yellow, red, and black being most pre- 

 dominant. 



Oceania turrita is the only known Medusid in 

 which vesicles and pigment-spots co-exist (^i^. 23). 



In the umbrella of the Lucernaridce, both 

 vesicles and pigment-spots seem to become united 

 into a single organ, termed the ' lithocyst.' These 

 marginal bodies are protected, externally, by a 

 sort of hood, and present often a very complex 

 arrangement. Most frequently they occur as 

 ovate vesicles, mounted on short, hollow stalks, 

 each of which communicates by means of a canal 

 with one of the radiating vessels of the umbrella. 

 Within, the vesicle is delicately ciliated, and en- 

 closes at its free extremity a broad, thin-walled, 

 oval sacculus, packed with a number of six-sided 

 crystalline prisms, obliquely truncated at each 

 end. 



