164 LECTURE IX. 



upon tlie ultimate ramifications at the margins and clavate ends of 

 the proboscis. These pores are, in truth, the commencement of the 

 nutritive system ; they are, in this respect, analogous to the numerous 

 polype-mouths of the compound coral zoophyte* ; but in the Rhizo- 

 stome a common central sac is interposed between the ingestive 

 conduits and the vascular or chylaqueous system of the body. Minute 

 animalcules, or the juices of a decomposing and dissolving larger 

 animal, are absorbed by these pores, and are conveyed, more or less 

 digested "in transitu," by the successively uniting canals to the 

 central cavity. Digestion being completed, the chyle passes at once 

 into the vascular system, which is in fact a continuation and rami- 

 fication of the digestive cavity. The nutrient fluid, with sea-water, 

 passes by vessels (e), which radiate from that cavity, to a beautiful 

 net- work (f,/) of large capillaries, which is spread upon the under 

 surface of the margin of the disc. The elegance and precision with 

 which the injections of Hunter have demonstrated this network in 

 his pre|)arations cannot be surpassed ; but it is to Cuvier that we owe 

 the first description of the very remarkable and interesting system of 

 nutrition in the Rhizostome.f 



The rich development and reticular disposition of this part of the 

 vascular system, in which the circulating fluids are exposed to the 

 surrounding medium in a state of minute subdivision, upon that 

 surface of the body which rests upon the water, indicate that the 

 respiratory interchange of the gases, and the absorption of the oxygen 

 from the air contained in the sea water, take place principally at this 

 vascular surface of the gelatinous disc ; and that Hunter is correct in 

 placing it amongst the series of respiratory organs. It stands, indeed, 

 at the lowest step of that series, since the organ is not specially 

 eliminated; but only indicated or sketched out, as it were, by a modi- 

 fication of part of the common integuments. 



The CyancBa aurita, another species of our coasts, exemplifies 



* LXXXIV. (1843.) p. 104. Mr. Huxley, who describes the oral pores in 

 Mhizostoma as oiJening obliquely in a channel fonned by two irritable marginal 

 membranes, states that in Cephcea the membranes unite in front of and behind 

 each pore " so as to fonn a distinct polype-like cell." The outer membrane and 

 the richly-ciliated lining membrane of the digestive canals unite at the f/ee edge of 

 the fold, and are there produced into tentacles. Belie"\ing the business of diges- 

 tion to be perfoiTned in the minute dilatations of the beginning of the canals col- 

 lected upon the edges and extremities of the ramuscules of the proboscis, he 

 carries out the analogy suggested in the text, and observes "that the Rhizostomida;, 

 quoad their digestive system, have the same relation to the monostome Medusae, as 

 the Sertularian Polypes have to the Hydra?, or the coralline Polypes to the Actinite," 

 Phil. Trans. 1849., p. 414. 



t CXXXVIII. p. 436. 



