110 HYDROZOA. 



large canals take their origin, and run in the same direction ; of these, 

 the two lateral ones are simple and unbranched, but that in the centre 

 ramified dichotomously. These sixteen large vascular trunks, together 

 with all their numerous ramifications, sometimes anastomotically united, 

 ultimately terminate in a wide circular vessel that surrounds the 

 margin of the disk. The nutrient canals are situated beneath the 

 inner membrane, described above, whereby they are partially enclosed 

 and supported. 



(282.) Before closing our description of the alimentary system of the 

 Pulmonigrade Acalephae, we must mention some accessory organs, of 

 recent discovery, which are in connexion with it. Eschscholtz* de- 

 scribes a series of elongated granular bodies, placed in little depressions 

 around the margin of the disk, which seem to be of a glandular nature, 

 and apparently communicate by means of minute tubes with the nu- 

 trient canals : these he regards as the rudiments of a biliary system. 

 Other observers assign a similar office to a cluster of blind sacculi or 

 caeca, which are connected in some species with the commencement of 

 the radiating tubes ; it is, however, scarcely necessary to observe that 

 such surmises relative to the function of minute parts are but little 

 satisfactory. 



(283.) Prior to the publication of Ehrenberg's important researches 

 relative to the anatomy of the Cyanea aurita't, it was generally 

 believed that in the Pulmonigrade Medusae the alimentary canals were 

 unprovided with any excrementitious orifices ; these, however, were 

 discovered by the illustrious Prussian observer, occupying the situations 

 indicated by eight dark-brown-coloured spots situated at equal dis- 

 tances around the margin of the disk, and which had previously been 

 suspected to be the analogues of a biliary organ. By keeping the 

 living Medusae for some time in sea-water deeply coloured with indigo, 

 and thus causing all the ramifications of the alimentary apparatus to 

 become filled with the coloured fluid, while the rest of the body re- 

 mained transparent and colourless, it appeared that, opposite each of 

 the above-mentioned spots, the circular marginal canal into which the 

 nutritive tubes, radiating from the stomach, empty themselves becomes 

 dilated into a sort of cloacal cavity, in which the debris of digested 

 materials, such as the shells of minute Conchifera, Rotifera, Bacil- 

 laria, &c., were easily distinguishable ; from each of these cloacal dila- 

 tations, canals can readily be traced communicating with the exterior ; 

 and on irritating the living animal, it is easy to witness the discharge 

 of excrementitious matter through the eight marginal orifices of the 

 disk. 



(284.) A distinct movement is frequently perceptible in the interior 



* System der Acalephen. Berlin, 1822. Annalesdes Sciences Naturelles,vol.xxviii. 

 p. 251. 



f Abhandl. der Konigl. Akad. der Wissenschaften zu Berlin, 1835. 



