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ite i 

 11 



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s 



thej 

 rfact 



t abmi 



rcular: 





ad' 



ii 



HYDKOZOA. 



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 nectosac, beneath the walls of which they 

 are prolonged until they reach a circular vessel 

 surrounding its margin (fig. 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 

 °f I4 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 



in 



swimming 



of a nectocalyx or umbrella, is usually furnished 



around its margin with a number of supposed 



. organs of sense, known as the marginal bodies. 



23) l For the best account yet given of their structure 



and relations, we are indebted to the researches of 



Will „ 6 ^„ aixx . 



Two kinds of these bodies are found in the 



■ a U of' Medusidce 



e vesicles,' and < pigment-spots 



The vesicles are thin-walled, rounded or 'ovate, 

 fthisU sacs, lined internally with an epithelial layer, and 



0- 



