ACALEPHiE. 



165 



another (monostome) type of the digestive system. The homologue 

 of the central cavity in RJiizostoma, and which in Cyancea must be 

 the digestive sac, consists of four cavities, from which as many short 

 oesophageal tubes are continued to their commencement, upon the 

 under surface of the body, by a single quadrangular mouth (Jtg. 76, a) 



with the angles prolonged 

 into four tentacles which 

 consist of a solid hyaline 

 axis, with two fimbriated 

 membranes along their 

 under surface. Sixteen 

 canals radiate from the 

 central cavities, eight of 

 which (6, b) form, by their 

 ramifications, the systems 

 of nutrient and respiratory 

 capillaries and terminate in 

 a circular canal near the 

 margin of the disc ; whilst 

 the alternate eight termi- 

 nate without dividing, each 

 Cyanaa. by a minute excretory o;i- 



fice (c) at the margin of the disc. 



We must suppose the mouths of these excretory vessels to be 

 endowed with an irritability of a different kind from that of the 

 nutrient canals, like the mouths of the different cavities of a ruminat- 

 ing stomach. For, as the orifices of the third and fourth stomachs 

 contract upon the coarse unmasticated food, whilst those of the first 

 and second open to receive it, and close when it is presented to them 

 in its remasticated state, so the nutrient diverticula of the stomach of 

 the Cyancea receive the digested and exclude the excrementitious 

 part of the food, which passes along the efferent canals and is thus 

 rejected from the system. 



The discovery of this condition of the nutrient apparatus in the 

 Cyancea aurita is due to the ingenuity and perseverance of Prof". 

 Ehrenberg, who induced the living animals to swallow indigo with 

 their food. He has represented the canals so injected, in the ela- 

 borate plates of his memoir on the anatomy of this species.* Prof. 

 Wagner f saw the currents of the nutrient fluid in the vascular 

 system of the Oceania : they were produced, not by peristaltic con- 

 traction of the canals, but by the vibration of the cilia lining them. 

 In the "bare-eyed" Medusae digestion is chiefly if not exclusively 



CXL. p. 189. taf. L and iv. lig. 2., ?. 



t cxLir. 



M 3 



