ORGANS OF SECRETION. 579 



series of gemmiparous points of the common fleshy sub- 

 stance. 



Most of the acalepha exhibit tubular prolongations, sim- 

 ple or ramified, extending from the periphery of their great 

 gastric sac, by which the absorbent and secreting internal 

 surface is increased, and the nutriment is distributed to a 

 greater distance from the central cavity. These become 

 more circumscribed tubular coeca, like biliary tubuli, in the 

 stomach of physalia, carybdea, and other genera, and the 

 analogies of these gastric coeca to the liver, have long been 

 suggested by Eschscholtz, Edwards, and others. The mar- 

 ginal, cylindrical, opaque, granular bodies around the disk 

 of many medusa, have been regarded as the liver of these 

 animals. Most of the species are known to secrete upon 

 their surface a matter of a highly acrid, poisonous, and 

 stinging quality, which appears to be furnished by every 

 point of their surface, and which exerts peculiarly irritating 

 effects on the human skin and other parts. I have expe- 

 rienced the violent inflammation and burning torture resulting 

 from its application to the surface of the eye. Another de- 

 fensive secretion of these animals, is the luminous material 

 which appears to be connected with the mucous secretion of 

 their surface, and to pervade nearly the whole class. The 

 brilliant hues emitted by day, and the vivid light emitted in 

 the dark, by beroes and other ciliograde forms, appear to be 

 confined to the large vibratile cilia. The physograde species, 

 secrete a gaseous fluid into their air-sac to assist in swim- 

 ming, and some others as physophora, porpita, and velella, 

 secrete a firm horny skeleton to support their soft parts. 

 Some secrete a bright blood-red matter into their long slender 

 tentacula, and the velellce are covered with a soft substance 

 of a deep violet-blue colour. 



The small follicular coeca, sometimes ramified, which open 

 into the digestive canal of holothuria, and into the wide 

 gastric cavity of stellerida, are considered by Chiaje as con- 

 stituting the liver of the echinoderma ; but the hepatic tubuli 

 of higher and lower allied classes have still more close analo- 

 gies with the ordinary large ramified gastric coeca extending 

 into the rays of asterias, and in other genera. If a difference 

 of function existed in these gastric appendices, the smaller 

 isolated follicles would rather represent a pancreas in these 



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