38 HYDROZOA. 



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 Gevyonia 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 tiirrita is the only known Medusid in 

 which vesicles and pigment-spots co-exist (j^^. 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 

 Avith 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. 



