HYDROZOA. 37 



the thickness of the roof of the nectocalyx there 

 occurs, also, a second cavity, from which issue four 

 or more canals, having no direct communication 

 with the uectosac, beneath the walls of which they 

 are prolonged until they reach a circular vessel 

 surrounding its margin (/i^. 23). The substance 

 of the nectocalyx consists chiefly of ectoderm, but 

 a continuation of the endoderm lines the ' necto- 

 calycine canals ' and the cavity from which they 

 arise. 



The function of the nectocalyx is sufficiently 

 simple. By the rhythmic contractions of its mus- 

 cular lining, the water within the nectosac is ex- 

 pelled, and the organism moves in a contrary 

 direction. 



The umbrella of the Lucernaridce bears some 

 resemblance to a nectocalyx, and, like it, may 

 perform the function of a natatorial organ. It 

 is easily distinguished by the absence of a veil : 

 its size, also, in the great majority of cases, is 

 much more considerable (figs, 7 and 25). 



7. IVervous System and Or$;ans of Sense. 



— The swimming organ, whether it take the form 

 of a nectocalyx or umbrella, is usually furnished 

 around its margin with a number of supposed 

 organs of sense, known as the marginal bodies. 

 For the best account yet given of their structure 

 and relations, we are indebted to the researches of 

 Will and Gregenbaur. 



Two kinds of these bodies are found in the 

 Medusidce, — 'vesicles,' and 'pigment-spots.' 



The vesicles are thin-walled, rounded or ovate, 

 sacs, lined internally with an epithelial layer, and 

 D 3 



