260 Dr. T. Williams on the Mechanism of Aquatic 



questionably with those floating on the fluid of the peritoneal ca- 

 vity. The vessel may be readily isolated and placed detachedly 

 under the eye of the microscope. The blood-corpuscles while in 

 the vessel continue in ceaseless motion ; this motion instantly 

 ceases upon their escape from the vessel. Vibratile cilia may be 

 actually seen on the internal lining membrane of the vessel : 

 the motion is due to their vibration. Miiller and Quatrefages 

 and Dr. Peters incidentally refer to this phsenomenon; by 

 neither of these observers has it been referred to its true cause. 

 In no other class of animals are the internal surfaces of true blood- 

 vessels lined with cilia; it is because in the Echinoderms the 

 blood- system is rudimentarily formed, that this aberrant phseno- 

 menon is intelligible. It is in the Echinoderms that the blood- 

 proper system Jirst appears in the zoological series. Nature's 

 first effort is imperfect ; the system is not independent of, closed 

 off from, the other fluid systems of the organism ; it derives its 

 contents from those of the visceral cavity. The water-vascular 

 system is exclusively locomotive and suctorial in function; it 

 nowhere exhibits connexion with the branchial organs. Its fluid 

 contents however indubitably communicate in some manner with 

 those of the peritoneal cavity ; the microscope proves them to 

 be identical. In the Holothuridan genera these admit of more 

 complete solution. In them the three systems exist under a 

 pronounced form. The blood-system is more highly developed 

 than in the inferior Echinoderms ; it supplies branches to the 

 tentacles, to the integuments, and forms a mesenteric plexus. 

 The cavity of the body is notwithstanding filled by a highly cor- 

 pusculated fluid, which penetrates into the hollow of the ten- 

 tacles, and comes into near contact with the surrounding ele- 

 ment through the fenestra of the integument. In this genus 

 then the two fluid systems are separately submitted to the pro- 

 cess of aeration. The parietes of the tentacles bear a ramifica- 

 tion of true blood-vessels. Their hollow axes are filled with 

 chylaqueous fluid. But in the Holothuridse a fourth system of 

 fluids is superadded — that of the respiratory tree. The meaning 

 of the respiratory tree is even now enigmatical ; it consists of a 

 csecal, subdivided tube, filled with sea water, and communicating 

 openly with the cloaca. It floats in the fluid of the visceral cavity. 

 Its parietes are not supplied by a plexiform vascular system. The 

 plexus formed by the blood-vessels lies in the folds of the me- 

 sentery, and belongs to the intestine ; it exhibits no connexion 

 with the respiratory tree. What then can be the signification of 

 this excentric and paradoxical organ ? Every comparative phy- 

 siologist from Tiedemann to Miiller has recognised in it a true 

 respiratory organ ; but in what possible manner can it accomplish 

 such a function ? The sea water admitted into its interior can- 



